diff -u b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageStyleConfigEntityUnitTest.php b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageStyleConfigEntityUnitTest.php
--- b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageStyleConfigEntityUnitTest.php
+++ b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageStyleConfigEntityUnitTest.php
@@ -91,7 +91,7 @@
     $entity = new ResponsiveImageStyle(array('breakpoint_group' => 'test_group'));
     $entity->setBreakpointGroup('test_group');
     $entity->addImageStyleMapping('test_breakpoint', '1x', array('image_style'=>'test_style'));
-    $entity->addImageStyleMapping('test_breakpoint', '2x', array());
+    $entity->addImageStyleMapping('test_breakpoint', '2x', array('image_style'=>''));
 
     $this->breakpointManager->expects($this->any())
       ->method('getGroupProviders')