diff --git a/modules/schemadotorg_additional_type/src/SchemaDotOrgAdditionalTypeManagerInterface.php b/modules/schemadotorg_additional_type/src/SchemaDotOrgAdditionalTypeManagerInterface.php
index 14954a22e26c2059af4500a9fc0b5db6fce0f8d1..1c3b5594c93c898545b11282b68b02536cd0ff92 100644
--- a/modules/schemadotorg_additional_type/src/SchemaDotOrgAdditionalTypeManagerInterface.php
+++ b/modules/schemadotorg_additional_type/src/SchemaDotOrgAdditionalTypeManagerInterface.php
@@ -16,12 +16,12 @@ interface SchemaDotOrgAdditionalTypeManagerInterface {
   /**
    * Add new field mapping option.
    */
-  const ADD_FIELD = SchemaDotOrgEntityFieldManagerInterface::ADD_FIELD;
+  const string ADD_FIELD = SchemaDotOrgEntityFieldManagerInterface::ADD_FIELD;
 
   /**
    * The additional type field name suffix.
    */
-  const FIELD_NAME_SUFFIX = '_type';
+  const string FIELD_NAME_SUFFIX = '_type';
 
   /**
    * Alter Schema.org mapping entity default values.
diff --git a/modules/schemadotorg_content_model_documentation/src/SchemaDotOrgContentModelDocumentationManagerInterface.php b/modules/schemadotorg_content_model_documentation/src/SchemaDotOrgContentModelDocumentationManagerInterface.php
index fce026dcff3dff282c299e6551224bd667f88f7f..64038feae736ff95f047f6adfba7575e4804b655 100644
--- a/modules/schemadotorg_content_model_documentation/src/SchemaDotOrgContentModelDocumentationManagerInterface.php
+++ b/modules/schemadotorg_content_model_documentation/src/SchemaDotOrgContentModelDocumentationManagerInterface.php
@@ -15,7 +15,7 @@ interface SchemaDotOrgContentModelDocumentationManagerInterface {
   /**
    * A mapping of Schema.org mapping types to documentable entities.
    */
-  public const DOCUMENTABLE_ENTITIES = [
+  public const array DOCUMENTABLE_ENTITIES = [
     'taxonomy_term' => 'taxonomy',
     'block_content' => 'block',
     'node' => 'node',
diff --git a/modules/schemadotorg_custom_field/schemadotorg_custom_field.module b/modules/schemadotorg_custom_field/schemadotorg_custom_field.module
index 5f80a378222fdaf89bc72d05f1ec945cbff295ca..f37283f91979f9b823bf763625a2a3e4958bbdfb 100644
--- a/modules/schemadotorg_custom_field/schemadotorg_custom_field.module
+++ b/modules/schemadotorg_custom_field/schemadotorg_custom_field.module
@@ -163,7 +163,12 @@ entity_type_id--bundle--SchemaType--propertyName:
   schema_type: SchemaType
   schema_properties:
     propertyName01:
-      data_type: string",
+      data_type: string
+entity_type_id--field_custom_name:
+  schema_properties:
+    propertyName01:
+      data_type: string
+",
   ];
   $form['schemadotorg_custom_field']['default_format'] = [
     '#type' => 'select',
diff --git a/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManager.php b/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManager.php
index b5cab9fd0eedc8bfd90b9dbff0d29e992ed0ae4b..44de9f22d7db7213710ecce3a8863abeae0939a9 100644
--- a/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManager.php
+++ b/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManager.php
@@ -166,6 +166,7 @@ class SchemaDotOrgCustomFieldManager implements SchemaDotOrgCustomFieldManagerIn
     $default_schema_properties = $this->getDefaultProperties(
       entity_type_id: $field_values['entity_type'],
       bundle: $field_values['bundle'],
+      field_name: $field_values['field_name'],
       schema_type: $schema_type,
       schema_property: $schema_property,
     );
@@ -278,7 +279,7 @@ class SchemaDotOrgCustomFieldManager implements SchemaDotOrgCustomFieldManagerIn
   /**
    * {@inheritdoc}
    */
-  public function hasDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): bool {
+  public function hasDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $field_name = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): bool {
     return (bool) $this->getDefaultProperties(
       entity_type_id: $entity_type_id,
       bundle: $bundle,
@@ -290,12 +291,13 @@ class SchemaDotOrgCustomFieldManager implements SchemaDotOrgCustomFieldManagerIn
   /**
    * {@inheritdoc}
    */
-  public function getDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): ?array {
+  public function getDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $field_name = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): ?array {
     $default_schema_properties = $this->configFactory->get('schemadotorg_custom_field.settings')
       ->get('default_schema_properties') ?? [];
     $parts = [
       'entity_type_id' => $entity_type_id,
       'bundle' => $bundle,
+      'field_name' => $field_name,
       'schema_type' => $schema_type,
       'schema_property' => $schema_property,
     ];
diff --git a/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManagerInterface.php b/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManagerInterface.php
index 082464077540802457c7de83ec68b493e5cf76e7..6ba2756ca73392c072591ed3fa432b75a4b50ecf 100644
--- a/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManagerInterface.php
+++ b/modules/schemadotorg_custom_field/src/SchemaDotOrgCustomFieldManagerInterface.php
@@ -81,6 +81,8 @@ interface SchemaDotOrgCustomFieldManagerInterface {
    *   The entity type ID.
    * @param string|null $bundle
    *   The bundle.
+   * @param string|null $field_name
+   *   The field name.
    * @param string|null $schema_type
    *   The Schema.org type.
    * @param string|null $schema_property
@@ -89,7 +91,7 @@ interface SchemaDotOrgCustomFieldManagerInterface {
    * @return bool
    *   TRUE if a Schema.org property is a custom field.
    */
-  public function hasDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): bool;
+  public function hasDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $field_name = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): bool;
 
   /**
    * Retrieves the default custom field properties.
@@ -98,6 +100,8 @@ interface SchemaDotOrgCustomFieldManagerInterface {
    *   The entity type ID.
    * @param string|null $bundle
    *   The bundle.
+   * @param string|null $field_name
+   *   The field name.
    * @param string|null $schema_type
    *   The Schema.org type.
    * @param string|null $schema_property
@@ -106,7 +110,7 @@ interface SchemaDotOrgCustomFieldManagerInterface {
    * @return array|null
    *   The default custom field properties.
    */
-  public function getDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): ?array;
+  public function getDefaultProperties(?string $entity_type_id = NULL, ?string $bundle = NULL, ?string $field_name = NULL, ?string $schema_type = NULL, ?string $schema_property = NULL): ?array;
 
   /**
    * Prepare a property's field data before the Schema.org mapping form.
diff --git a/modules/schemadotorg_custom_field/tests/src/Kernel/SchemaDotOrgCustomFieldManagerKernelTest.php b/modules/schemadotorg_custom_field/tests/src/Kernel/SchemaDotOrgCustomFieldManagerKernelTest.php
index fd4f85ef7dbca85537f514b6fb62bf5c94d87b6e..3ccdf7ff253d473a8226cec7df1cd5e02fb437cb 100644
--- a/modules/schemadotorg_custom_field/tests/src/Kernel/SchemaDotOrgCustomFieldManagerKernelTest.php
+++ b/modules/schemadotorg_custom_field/tests/src/Kernel/SchemaDotOrgCustomFieldManagerKernelTest.php
@@ -636,6 +636,54 @@ class SchemaDotOrgCustomFieldManagerKernelTest extends SchemaDotOrgEntityKernelT
       ],
     ];
     $this->assertEquals($expected_settings, $settings['field_settings']);
+
+    /* ********************************************************************** */
+    // Custom.
+    /* ********************************************************************** */
+
+    $this->config('schemadotorg_custom_field.settings')->set('default_schema_properties.field_custom', [
+      'schema_properties' => [
+        'name' => ['data_type' => 'string'],
+        'value' => ['data_type' => 'string'],
+      ],
+    ])->save();
+
+    $this->createSchemaEntity('node', 'Thing', [
+      'properties' => [
+        'field_custom' => [
+          'type' => 'custom',
+          'name' => 'field_custom',
+          'label' => 'Custom',
+        ],
+      ],
+    ]);
+
+    // Check custom field storage columns.
+    /** @var \Drupal\field\FieldStorageConfigInterface|null $field_storage_config */
+    $field_storage_config = FieldStorageConfig::loadByName('node', 'field_custom');
+    $expected_settings = [
+      'columns' => [
+        'name' => [
+          'name' => 'name',
+          'type' => 'string',
+          'max_length' => '255',
+          'unsigned' => 0,
+          'precision' => '10',
+          'scale' => '2',
+          'datetime_type' => 'datetime',
+        ],
+        'value' => [
+          'name' => 'value',
+          'type' => 'string',
+          'max_length' => '255',
+          'unsigned' => 0,
+          'precision' => '10',
+          'scale' => '2',
+          'datetime_type' => 'datetime',
+        ],
+      ],
+    ];
+    $this->assertEquals($expected_settings, $field_storage_config->getSettings());
   }
 
 }
diff --git a/modules/schemadotorg_descriptions/src/Config/SchemaDotOrgDescriptionsConfigFactoryOverride.php b/modules/schemadotorg_descriptions/src/Config/SchemaDotOrgDescriptionsConfigFactoryOverride.php
index b688e6816f3c3af34974abf2d9554c507d06634b..839c6e5035cda10b0fe27f655b53c1b54e10607e 100644
--- a/modules/schemadotorg_descriptions/src/Config/SchemaDotOrgDescriptionsConfigFactoryOverride.php
+++ b/modules/schemadotorg_descriptions/src/Config/SchemaDotOrgDescriptionsConfigFactoryOverride.php
@@ -34,7 +34,7 @@ class SchemaDotOrgDescriptionsConfigFactoryOverride extends ConfigFactoryOverrid
   /**
    * The cache id.
    */
-  const CACHE_ID = 'schemadotorg_descriptions.override';
+  const string CACHE_ID = 'schemadotorg_descriptions.override';
 
   /**
    * Constructs a SchemaDotOrgDescriptionsConfigFactoryOverride object.
diff --git a/modules/schemadotorg_diagram/src/SchemaDotOrgDiagramBuilder.php b/modules/schemadotorg_diagram/src/SchemaDotOrgDiagramBuilder.php
index 04cb9a07f07e8f32895cc0e11ae82dee72976ba7..a8c850f0c34369d708bb7544096680a533852528 100644
--- a/modules/schemadotorg_diagram/src/SchemaDotOrgDiagramBuilder.php
+++ b/modules/schemadotorg_diagram/src/SchemaDotOrgDiagramBuilder.php
@@ -30,17 +30,17 @@ class SchemaDotOrgDiagramBuilder implements SchemaDotOrgDiagramBuilderInterface
   /**
    * Current node.
    */
-  const CURRENT_NODE = 'current';
+  const string CURRENT_NODE = 'current';
 
   /**
    * Parent node.
    */
-  const PARENT_NODE = 'parent';
+  const string PARENT_NODE = 'parent';
 
   /**
    * Child node.
    */
-  const CHILD_NODE = 'child';
+  const string CHILD_NODE = 'child';
 
   /**
    * Max depth for hierarchy.
diff --git a/modules/schemadotorg_epp/src/SchemaDotOrgEppManagerInterface.php b/modules/schemadotorg_epp/src/SchemaDotOrgEppManagerInterface.php
index 7ef10b78fe0eb1dc5498088c275486f41c799ac8..0bb7e08aa5d6c30b91868f7d1ff1314e146139e2 100644
--- a/modules/schemadotorg_epp/src/SchemaDotOrgEppManagerInterface.php
+++ b/modules/schemadotorg_epp/src/SchemaDotOrgEppManagerInterface.php
@@ -14,17 +14,17 @@ interface SchemaDotOrgEppManagerInterface {
   /**
    * Do not display node links.
    */
-  const HIDDEN = '';
+  const string HIDDEN = '';
 
   /**
    * Display node links as dropdown menu.
    */
-  const DROPDOWN = 'dropdown';
+  const string DROPDOWN = 'dropdown';
 
   /**
    * Display node links as buttons.
    */
-  const BUTTONS = 'buttons';
+  const string BUTTONS = 'buttons';
 
   /**
    * Alter field storage and field values before they are created.
diff --git a/modules/schemadotorg_field_group/src/SchemaDotOrgFieldGroupEntityDisplayBuilderInterface.php b/modules/schemadotorg_field_group/src/SchemaDotOrgFieldGroupEntityDisplayBuilderInterface.php
index d6eb7192f57e2eefaba9b23d2b35a5ada5917b6c..b6720e27a06be7d2c60912a4ced029e8894a239e 100644
--- a/modules/schemadotorg_field_group/src/SchemaDotOrgFieldGroupEntityDisplayBuilderInterface.php
+++ b/modules/schemadotorg_field_group/src/SchemaDotOrgFieldGroupEntityDisplayBuilderInterface.php
@@ -15,7 +15,7 @@ interface SchemaDotOrgFieldGroupEntityDisplayBuilderInterface {
   /**
    * Disabled field group patterns.
    */
-  const PATTERNS = [
+  const array PATTERNS = [
     ['entity_type_id'],
     ['entity_type_id', 'display_type'],
     ['entity_type_id', 'display_type', 'bundle'],
diff --git a/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdBuilderInterface.php b/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdBuilderInterface.php
index 732c40adf7ca5d2546c5d254fd63016fd86330c6..2e2a8194af8b77d27fb642d4150440749196806b 100644
--- a/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdBuilderInterface.php
+++ b/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdBuilderInterface.php
@@ -18,22 +18,22 @@ interface SchemaDotOrgJsonLdBuilderInterface {
   /**
    * Cache contexts when building JSON-LD from a route match.
    */
-  public const ROUTE_MATCH_CACHE_CONTEXTS = ['user.permissions', 'route'];
+  public const array ROUTE_MATCH_CACHE_CONTEXTS = ['user.permissions', 'route'];
 
   /**
    * Cache tags when building JSON-LD from a route match.
    */
-  public const ROUTE_MATCH_CACHE_TAGS = ['config:schemadotorg_jsonld.settings'];
+  public const array ROUTE_MATCH_CACHE_TAGS = ['config:schemadotorg_jsonld.settings'];
 
   /**
    * Cache contexts when building JSON-LD for an entity.
    */
-  public const ENTITY_CACHE_CONTEXTS = ['user.permissions'];
+  public const array ENTITY_CACHE_CONTEXTS = ['user.permissions'];
 
   /**
    * Cache contexts when building JSON-LD for an entity.
    */
-  public const ENTITY_CACHE_TAGS = ['config:schemadotorg_jsonld.settings', 'config:schemadotorg_mapping_list'];
+  public const array ENTITY_CACHE_TAGS = ['config:schemadotorg_jsonld.settings', 'config:schemadotorg_mapping_list'];
 
   /**
    * Build JSON-LD for a route.
diff --git a/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdManagerInterface.php b/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdManagerInterface.php
index 83a8ea7136c6db2e0d77d28950e3a49ac2ed1dc1..f722aa2ff2507ea4a4420c088fd894214bfcd032 100644
--- a/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdManagerInterface.php
+++ b/modules/schemadotorg_jsonld/src/SchemaDotOrgJsonLdManagerInterface.php
@@ -18,22 +18,22 @@ interface SchemaDotOrgJsonLdManagerInterface {
   /**
    * Entity reference display url.
    */
-  const ENTITY_REFERENCE_DISPLAY_URL = 'url';
+  const string ENTITY_REFERENCE_DISPLAY_URL = 'url';
 
   /**
    * Entity reference display label.
    */
-  const ENTITY_REFERENCE_DISPLAY_LABEL = 'label';
+  const string ENTITY_REFERENCE_DISPLAY_LABEL = 'label';
 
   /**
    * Entity reference display entity.
    */
-  const ENTITY_REFERENCE_DISPLAY_ENTITY = 'entity';
+  const string ENTITY_REFERENCE_DISPLAY_ENTITY = 'entity';
 
   /**
    * Entity reference display none.
    */
-  const ENTITY_REFERENCE_DISPLAY_NONE = 'none';
+  const string ENTITY_REFERENCE_DISPLAY_NONE = 'none';
 
   /**
    * Get an entity's canonical route match.
diff --git a/modules/schemadotorg_jsonld_endpoint/src/Routing/SchemaDotOrgJsonLdEndpointRoutes.php b/modules/schemadotorg_jsonld_endpoint/src/Routing/SchemaDotOrgJsonLdEndpointRoutes.php
index 8534d75f706bd41df68b3c65d86b9076d29b5234..2c972a03b98ed01dab7f92d849fa920a7f5ec972 100644
--- a/modules/schemadotorg_jsonld_endpoint/src/Routing/SchemaDotOrgJsonLdEndpointRoutes.php
+++ b/modules/schemadotorg_jsonld_endpoint/src/Routing/SchemaDotOrgJsonLdEndpointRoutes.php
@@ -20,7 +20,7 @@ class SchemaDotOrgJsonLdEndpointRoutes implements ContainerInjectionInterface {
   /**
    * A key with which to flag a route as belonging to the Schema.org JSON-LD endpoint module.
    */
-  const JSONLD_ROUTE_FLAG_KEY = '_is_schemadotorg_jsonld_endpoint';
+  const string JSONLD_ROUTE_FLAG_KEY = '_is_schemadotorg_jsonld_endpoint';
 
   /**
    * Constructs a SchemaDotOrgJsonLdEndpointRoutes object.
diff --git a/modules/schemadotorg_jsonld_preview/src/SchemaDotOrgJsonLdPreviewBuilderInterface.php b/modules/schemadotorg_jsonld_preview/src/SchemaDotOrgJsonLdPreviewBuilderInterface.php
index 9a8603cdfc7af489ded6fdb1e3f9a1686f8231bc..d089eb5a229b4af5532398a930412975d87cb5af 100644
--- a/modules/schemadotorg_jsonld_preview/src/SchemaDotOrgJsonLdPreviewBuilderInterface.php
+++ b/modules/schemadotorg_jsonld_preview/src/SchemaDotOrgJsonLdPreviewBuilderInterface.php
@@ -15,12 +15,12 @@ interface SchemaDotOrgJsonLdPreviewBuilderInterface {
   /**
    * Format Schema.org prevent via JSON-LD.
    */
-  const JSONLD = 'jsonld';
+  const string JSONLD = 'jsonld';
 
   /**
    * Format Schema.org prevent via data (table).
    */
-  const DATA = 'data';
+  const string DATA = 'data';
 
   /**
    * Build JSON-LD preview for a route.
diff --git a/modules/schemadotorg_layout_paragraphs/src/SchemaDotOrgLayoutParagraphsManagerInterface.php b/modules/schemadotorg_layout_paragraphs/src/SchemaDotOrgLayoutParagraphsManagerInterface.php
index 0c55e20b2eb15f34213ed5f2f944aecb4634798a..2c162669b85106a63a5fe0bd36fd72261b70c045 100644
--- a/modules/schemadotorg_layout_paragraphs/src/SchemaDotOrgLayoutParagraphsManagerInterface.php
+++ b/modules/schemadotorg_layout_paragraphs/src/SchemaDotOrgLayoutParagraphsManagerInterface.php
@@ -16,7 +16,7 @@ interface SchemaDotOrgLayoutParagraphsManagerInterface {
   /**
    * The layout paragraphs Schema.org property name.
    */
-  const PROPERTY_NAME = 'mainEntity';
+  const string PROPERTY_NAME = 'mainEntity';
 
   /**
    * Get layout paragraphs field machine name.
diff --git a/modules/schemadotorg_ui/src/Form/SchemaDotOrgUiMappingForm.php b/modules/schemadotorg_ui/src/Form/SchemaDotOrgUiMappingForm.php
index 1b9267e73d457dc78b69651d7c33d49d0ff2f263..3722d5090341559c305297323a51acef9aa23696 100644
--- a/modules/schemadotorg_ui/src/Form/SchemaDotOrgUiMappingForm.php
+++ b/modules/schemadotorg_ui/src/Form/SchemaDotOrgUiMappingForm.php
@@ -34,7 +34,7 @@ class SchemaDotOrgUiMappingForm extends EntityForm {
   /**
    * Add new field mapping option.
    */
-  public const ADD_FIELD = SchemaDotOrgEntityFieldManagerInterface::ADD_FIELD;
+  public const string ADD_FIELD = SchemaDotOrgEntityFieldManagerInterface::ADD_FIELD;
 
   /**
    * The service container.
diff --git a/src/Element/SchemaDotOrgAutocomplete.php b/src/Element/SchemaDotOrgAutocomplete.php
index f095270f48388437473b40ddf822eeb32608adc8..235fb9d0f44ba618daf15a84f2505c81cf340cea 100644
--- a/src/Element/SchemaDotOrgAutocomplete.php
+++ b/src/Element/SchemaDotOrgAutocomplete.php
@@ -22,12 +22,12 @@ class SchemaDotOrgAutocomplete extends Textfield {
   /**
    * Schema.org type.
    */
-  const SCHEMA_TYPES = SchemaDotOrgSchemaTypeManagerInterface::SCHEMA_TYPES;
+  const string SCHEMA_TYPES = SchemaDotOrgSchemaTypeManagerInterface::SCHEMA_TYPES;
 
   /**
    * Schema.org property.
    */
-  const SCHEMA_PROPERTIES = SchemaDotOrgSchemaTypeManagerInterface::SCHEMA_PROPERTIES;
+  const string SCHEMA_PROPERTIES = SchemaDotOrgSchemaTypeManagerInterface::SCHEMA_PROPERTIES;
 
   /**
    * Schema.org Thing without Enumerations.
@@ -36,7 +36,7 @@ class SchemaDotOrgAutocomplete extends Textfield {
    *
    * @see \Drupal\schemadotorg\Controller\SchemaDotOrgAutocompleteController::autocomplete
    */
-  const SCHEMA_THINGS = 'Thing';
+  const string SCHEMA_THINGS = 'Thing';
 
   /**
    * {@inheritdoc}
diff --git a/src/SchemaDotOrgEntityDisplayBuilderInterface.php b/src/SchemaDotOrgEntityDisplayBuilderInterface.php
index 5944676de37de59bf455dfa2ada83bb1bbec1c41..7d17bc78bad32bf162719e0c74ba8a9e1d266267 100644
--- a/src/SchemaDotOrgEntityDisplayBuilderInterface.php
+++ b/src/SchemaDotOrgEntityDisplayBuilderInterface.php
@@ -14,7 +14,7 @@ interface SchemaDotOrgEntityDisplayBuilderInterface {
   /**
    * Hide component from entity display.
    */
-  const COMPONENT_HIDDEN = 'schemadotorg_component_hidden';
+  const string COMPONENT_HIDDEN = 'schemadotorg_component_hidden';
 
   /**
    * Gets default field weights.
diff --git a/src/SchemaDotOrgEntityFieldManagerInterface.php b/src/SchemaDotOrgEntityFieldManagerInterface.php
index 0feb5cd1cb0c03945ceb034c954bbd40dffb4607..37e8118704863245de0761c6865fe0edc880d08e 100644
--- a/src/SchemaDotOrgEntityFieldManagerInterface.php
+++ b/src/SchemaDotOrgEntityFieldManagerInterface.php
@@ -14,7 +14,7 @@ interface SchemaDotOrgEntityFieldManagerInterface {
   /**
    * Add new field mapping option.
    */
-  const ADD_FIELD = '_add_';
+  const string ADD_FIELD = '_add_';
 
   /**
    * Determine if a field exists.
diff --git a/src/SchemaDotOrgInstaller.php b/src/SchemaDotOrgInstaller.php
index 69dc0d34d67d03a5d7f7ac111343ae5f661f2e3a..e0f4e09048bf40c976db0f89e6a3ef74a1aecaa0 100644
--- a/src/SchemaDotOrgInstaller.php
+++ b/src/SchemaDotOrgInstaller.php
@@ -35,7 +35,7 @@ class SchemaDotOrgInstaller implements SchemaDotOrgInstallerInterface {
   /**
    * Schema.org version.
    */
-  const VERSION = '28.0';
+  const string VERSION = '28.0';
 
   /**
    * Constructs a SchemaDotOrgInstaller object.
diff --git a/src/SchemaDotOrgNamesInterface.php b/src/SchemaDotOrgNamesInterface.php
index 9614428a8e0f448c3437b1a0ce61b9559f3fb455..add2a56891b09383f6d876989f8ec5125abce5bf 100644
--- a/src/SchemaDotOrgNamesInterface.php
+++ b/src/SchemaDotOrgNamesInterface.php
@@ -12,7 +12,7 @@ interface SchemaDotOrgNamesInterface {
   /**
    * Default prefix for Schema.org table and field names.
    */
-  const DEFAULT_PREFIX = 'schema_';
+  const string DEFAULT_PREFIX = 'schema_';
 
   /**
    * Gets the field prefix for Schema.org properties.
diff --git a/src/SchemaDotOrgSchemaTypeManagerInterface.php b/src/SchemaDotOrgSchemaTypeManagerInterface.php
index 4cafc3762f560f01f8d2a989de2a917792b93b15..d63127fd4202355b25c4fc498dc92bcb0bfd1608 100644
--- a/src/SchemaDotOrgSchemaTypeManagerInterface.php
+++ b/src/SchemaDotOrgSchemaTypeManagerInterface.php
@@ -22,17 +22,17 @@ interface SchemaDotOrgSchemaTypeManagerInterface {
   /**
    * Schema.org type.
    */
-  const SCHEMA_TYPES = 'types';
+  const string SCHEMA_TYPES = 'types';
 
   /**
    * Schema.org property.
    */
-  const SCHEMA_PROPERTIES = 'properties';
+  const string SCHEMA_PROPERTIES = 'properties';
 
   /**
    * The Schema.org base URI.
    */
-  const URI = 'https://schema.org/';
+  const string URI = 'https://schema.org/';
 
   /**
    * Gets Schema.org type or property URI.