diff -u b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
--- b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
+++ b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
@@ -116,7 +116,7 @@
    */
   public static function defaultSettings() {
     return [
-      'responsive_image_style' => '',
+      'responsive_image_style' => NULL,
       'image_link' => '',
     ] + parent::defaultSettings();
   }
diff -u b/core/modules/responsive_image/tests/src/Functional/ViewsIntegrationTest.php b/core/modules/responsive_image/tests/src/Functional/ViewsIntegrationTest.php
--- b/core/modules/responsive_image/tests/src/Functional/ViewsIntegrationTest.php
+++ b/core/modules/responsive_image/tests/src/Functional/ViewsIntegrationTest.php
@@ -15,6 +15,11 @@
 class ViewsIntegrationTest extends ViewTestBase {
 
   /**
+   * The responsive image style ID to use.
+   */
+  const RESPONSIVE_IMAGE_STYLE_ID = 'responsive_image_style_id';
+
+  /**
    * {@inheritdoc}
    */
   public static $modules = ['views', 'views_ui', 'responsive_image', 'field', 'image', 'file', 'entity_test', 'breakpoint', 'responsive_image_test_module'];
@@ -33,11 +38,11 @@
     $this->enableViewsTestModule();
 
     // Create a responsive image style.
-    ResponsiveImageStyle::create([
-      'id' => 'foo',
+    $responsive_image_style = ResponsiveImageStyle::create([
+      'id' => self::RESPONSIVE_IMAGE_STYLE_ID,
       'label' => 'Foo',
       'breakpoint_group' => 'responsive_image_test_module',
-    ])->save();
+    ]);
     // Create an image field to be used with a responsive image formatter.
     FieldStorageConfig::create([
       'type' => 'image',
@@ -50,6 +55,22 @@
       'field_name' => 'bar',
     ])->save();
 
+    $responsive_image_style
+      ->addImageStyleMapping('responsive_image_test_module.mobile', '1x', [
+        'image_mapping_type' => 'image_style',
+        'image_mapping' => 'thumbnail',
+      ])
+      ->addImageStyleMapping('responsive_image_test_module.narrow', '1x', [
+        'image_mapping_type' => 'image_style',
+        'image_mapping' => 'medium'
+      ])
+      // Test the normal output of mapping to an image style.
+      ->addImageStyleMapping('responsive_image_test_module.wide', '1x', [
+        'image_mapping_type' => 'image_style',
+        'image_mapping' => 'large',
+      ])
+      ->save();
+
     $admin_user = $this->drupalCreateUser(['administer views']);
     $this->drupalLogin($admin_user);
   }
@@ -64,6 +85,10 @@
     // Set the formatter to 'Responsive image'.
     $this->drupalPostForm(NULL, ['options[type]' => 'responsive_image'], 'Apply');
     $this->assertSession()->responseContains('Responsive image style field is required.');
+    $this->drupalPostForm(NULL, ['options[settings][responsive_image_style]' => self::RESPONSIVE_IMAGE_STYLE_ID], 'Apply');
+    $this->drupalGet('admin/structure/views/nojs/handler/entity_test_row/default/field/bar');
+    // Make sure the selected value is set.
+    $this->assertSession()->fieldValueEquals('options[settings][responsive_image_style]', self::RESPONSIVE_IMAGE_STYLE_ID);
   }
 
 }