Issue #2209071 by tstoeckler: StringItem should cast the 'length' schema value to (int).

core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php

@@ -44,7 +44,7 @@ public static function schema(FieldDefinitionInterface $field_definition) {
       'columns' => array(
         'value' => array(
           'type' => 'varchar',
-          'length' => $field_definition->getSetting('max_length'),
+          'length' => (int) $field_definition->getSetting('max_length'),
           'not null' => FALSE,
         ),
       ),