diff --git a/core/modules/migrate/tests/src/Unit/Plugin/migrate/ConfigurablePluginTraitTest.php b/core/modules/migrate/tests/src/Unit/Plugin/migrate/ConfigurablePluginTraitTest.php
index 1c59204365..6c5cb6fd42 100644
--- a/core/modules/migrate/tests/src/Unit/Plugin/migrate/ConfigurablePluginTraitTest.php
+++ b/core/modules/migrate/tests/src/Unit/Plugin/migrate/ConfigurablePluginTraitTest.php
@@ -14,7 +14,6 @@ class ConfigurablePluginTraitTest extends UnitTestCase {
 
   /**
    * Test a mock ConfigurablePluginTrait.
-   *
    */
   public function testConfigurableTrait() {
 
@@ -48,6 +47,8 @@ public function testConfigurableTrait() {
    *
    * Specifically test the way default and provided configurations are merged.
    *
+   * @param array $default_configuration
+   *   The default configuration to use for the trait.
    * @param array $test_configuration
    *   The configuration to test.
    * @param array $final_configuration
@@ -55,8 +56,8 @@ public function testConfigurableTrait() {
    *
    * @dataProvider configurableObjectDataProvider
    */
-  public function testConfigurableObject(array $test_configuration, array $final_configuration) {
-    $test_object = new ConfigurablePluginTestClass();
+  public function testConfigurableObject(array $default_configuration, array $test_configuration, array $final_configuration) {
+    $test_object = new ConfigurablePluginTestClass($default_configuration);
     $test_object->setConfiguration($test_configuration);
     $this->assertArrayEquals($final_configuration, $test_object->getConfiguration());
   }
@@ -70,6 +71,13 @@ public function testConfigurableObject(array $test_configuration, array $final_c
   public function configurableObjectDataProvider() {
     return [
       'Direct Override' => [
+        'default_configuration' => [
+          'default_key_1' => 'default_value_1',
+          'default_key_2' => [
+            'default_nested_key_1' => 'default_nested_value_1',
+            'default_nested_key_2' => 'default_nested_value_2',
+          ],
+        ],
         'test_configuration' => [
           'default_key_1' => 'override_value_1',
           'default_key_2' => [
@@ -86,6 +94,13 @@ public function configurableObjectDataProvider() {
         ],
       ],
       'Mixed Override' => [
+        'default_configuration' => [
+          'default_key_1' => 'default_value_1',
+          'default_key_2' => [
+            'default_nested_key_1' => 'default_nested_value_1',
+            'default_nested_key_2' => 'default_nested_value_2',
+          ],
+        ],
         'test_configuration' => [
           'override_key_1' => 'config_value_1',
           'default_key_2' => [
@@ -103,31 +118,75 @@ public function configurableObjectDataProvider() {
           'override_key_1' => 'config_value_1',
         ],
       ],
+      'indexed_override' => [
+        'default_configuration' => [
+          'config_value_1',
+          'config_value_2',
+          'config_value_3',
+        ],
+        'test_configuration' => [
+          'override_value_1',
+          'override_value_2',
+        ],
+        'final_configuration' => [
+          'override_value_1',
+          'override_value_2',
+          'config_value_3',
+        ]
+      ],
+      'indexed_override_complex' => [
+        'default_configuration' => [
+          'config_value_1',
+          'config_value_2',
+          'config_value_3',
+        ],
+        'test_configuration' => [
+          0 => 'override_value_1',
+          2 => 'override_value_3',
+        ],
+        'final_configuration' => [
+          'override_value_1',
+          'config_value_2',
+          'override_value_3',
+        ]
+      ]
+
     ];
   }
 
 }
 
 /**
- * A test object using ConfigurablePluginTrait.
+ * A test class using ConfigurablePluginTrait.
  */
 class ConfigurablePluginTestClass {
   use ConfigurablePluginTrait;
 
   /**
-   * Test defaults.
+   * A default configuration for the test class to return.
+   *
+   * @var array
+   */
+  protected $defaultConfiguration;
+
+  /**
+   * ConfigurablePluginTestClass constructor.
+   *
+   * @param array $default_configuration
+   *   The default configuration to return.
+   */
+  public function __construct(array $default_configuration) {
+    $this->defaultConfiguration = $default_configuration;
+  }
+
+  /**
+   * Return the provided test defaults.
    *
    * @return array
    *   The default configuration.
    */
   public function defaultConfiguration() {
-    return [
-      'default_key_1' => 'default_value_1',
-      'default_key_2' => [
-        'default_nested_key_1' => 'default_nested_value_1',
-        'default_nested_key_2' => 'default_nested_value_2',
-      ],
-    ];
+    return $this->defaultConfiguration;
   }
 
 }