diff --git a/core/modules/migrate/tests/src/Unit/process/MigrationTest.php b/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
index cd406cc..b62c1f9 100644
--- a/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
@@ -115,9 +115,9 @@ public function testSkipOnEmpty() {
    * @param array $destination_id_values
    *   The destination id(s) of the migration map.
    * @param string|array $source_value
-   *   The source value for the migration process plugin.
+   *   The source value(s) for the migration process plugin.
    * @param string|array $expected_value
-   *   The expected value of the migration process plugin.
+   *   The expected value(s) of the migration process plugin.
    */
   public function testSuccessfulLookup($source_id_values, $destination_id_values, $source_value, $expected_value) {
     $migration_plugin = $this->prophesize(MigrationInterface::class);
@@ -152,24 +152,50 @@ public function testSuccessfulLookup($source_id_values, $destination_id_values,
    */
   public function successfulLookupDataProvider() {
     return [
+      // Test data for scalar to scalar.
       [
+        // Source ID of the migration map.
         [1],
+        // Destination ID of the migration map.
         [3],
+        // Input value for the migration plugin.
         1,
+        // Expected output value of the migration plugin.
         3,
       ],
+      // Test data for scalar to array.
       [
+        // Source ID of the migration map.
         [1],
+        // Destination IDs of the migration map.
         [3, 'foo'],
+        // Input value for the migration plugin.
         1,
+        // Expected output values of the migration plugin.
         [3, 'foo'],
       ],
+      // Test data for array to scalar.
       [
+        // Source IDs of the migration map.
         [1, 3],
+        // Destination ID of the migration map.
         ['foo'],
+        // Input values for the migration plugin.
         [1, 3],
+        // Expected output value of the migration plugin.
         'foo',
       ],
+      // Test data for array to array.
+      [
+        // Source IDs of the migration map.
+        [1, 3],
+        // Destination IDs of the migration map.
+        [3, 'foo'],
+        // Input values for the migration plugin.
+        [1, 3],
+        // Expected output values of the migration plugin.
+        [3, 'foo'],
+      ],
     ];
   }