diff --git a/core/modules/media/tests/src/Kernel/MediaSourceTest.php b/core/modules/media/tests/src/Kernel/MediaSourceTest.php
index f59524fd82..0f07ab2646 100644
--- a/core/modules/media/tests/src/Kernel/MediaSourceTest.php
+++ b/core/modules/media/tests/src/Kernel/MediaSourceTest.php
@@ -431,10 +431,23 @@ public function testSourceConfigurationSubmit() {
     /** @var \Drupal\media\MediaSourceInterface $source */
     $source = $manager->createInstance('test', []);
     $source->submitConfigurationForm($form, $form_state);
-
     $expected = ['test_config_value' => 'Somewhere over the rainbow.', 'source_field' => 'field_media_test_1'];
     $this->assertEquals($expected, $source->getConfiguration(), 'Submitted values were saved correctly.');
 
+    // Try to save a NULL value.
+    $form_state->setValue('test_config_value', NULL);
+    $source->submitConfigurationForm($form, $form_state);
+    $expected['test_config_value'] = NULL;
+    $this->assertEquals($expected, $source->getConfiguration(), 'Submitted values were saved correctly.');
+
+    // Make sure that the config keys are determined correctly even if the
+    // existing value is NULL.
+    $form_state->setValue('test_config_value', 'Somewhere over the rainbow.');
+    $source->submitConfigurationForm($form, $form_state);
+    $expected['test_config_value'] = 'Somewhere over the rainbow.';
+    $this->assertEquals($expected, $source->getConfiguration(), 'Submitted values were saved correctly.');
+
+    // Make sure that a non-relevant value will be skipped.
     $form_state->setValue('not_relevant', 'Should not be saved in the plugin.');
     $source->submitConfigurationForm($form, $form_state);
     $this->assertEquals($expected, $source->getConfiguration(), 'Submitted values were saved correctly.');