diff --git a/core/modules/user/src/Plugin/migrate/process/ConvertTokens.php b/core/modules/user/src/Plugin/migrate/process/ConvertTokens.php
index 44da52f68a24bbe8abafc2d96aa13217f5e04032..b445e507ed41d724fc1e00e155bc96c4cb16b5d6 100644
--- a/core/modules/user/src/Plugin/migrate/process/ConvertTokens.php
+++ b/core/modules/user/src/Plugin/migrate/process/ConvertTokens.php
@@ -40,6 +40,13 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
       '!password' => '',
     );
 
+    // Given that our source is a database column that could hold a NULL
+    // value, sometimes that filters down to here. str_replace() cannot
+    // handle NULLs as the subject, so we reset to an empty string.
+    if (is_null($value)) {
+      $value = '';
+    }
+
     if (is_string($value)) {
       return str_replace(array_keys($tokens), $tokens, $value);
     }
diff --git a/core/modules/user/tests/src/Unit/Plugin/migrate/process/ConvertTokensTest.php b/core/modules/user/tests/src/Unit/Plugin/migrate/process/ConvertTokensTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..55c6559918a7e7038000eff2006aa5c554979304
--- /dev/null
+++ b/core/modules/user/tests/src/Unit/Plugin/migrate/process/ConvertTokensTest.php
@@ -0,0 +1,44 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\user\Unit\Plugin\migrate\process\ConvertTokensTest.
+ */
+
+namespace Drupal\Tests\user\Unit\Plugin\migrate\process;
+
+use Drupal\user\Plugin\migrate\process\ConvertTokens;
+use Drupal\Tests\migrate\Unit\process\MigrateProcessTestCase;
+
+/**
+ * Tests the ConvertTokens plugin.
+ *
+ * @group user
+ */
+class ConvertTokensTest extends MigrateProcessTestCase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->plugin = new ConvertTokens([], 'convert_tokens', []);
+  }
+
+  /**
+   * Tests conversion of user tokens.
+   */
+  public function testConvertTokens() {
+    $value = $this->plugin->transform('Account details for !username at !site', $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertEquals('Account details for [user:name] at [site:name]', $value);
+  }
+
+  /**
+   * Tests conversion of user tokens with a NULL value.
+   */
+  public function testConvertTokensNull() {
+    $value = $this->plugin->transform(NULL, $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertEquals('', $value);
+  }
+
+}