diff --git a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
index c01a03b..d97fd2a 100644
--- a/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
@@ -200,7 +200,7 @@ public function save(array $form, FormStateInterface $form_state) {
               'image_mapping_type' => 'sizes',
               'image_mapping' => array(
                 'sizes' => $image_style_mapping['sizes'],
-                'sizes_image_styles' => array_filter($image_style_mapping['sizes_image_styles']),
+                'sizes_image_styles' => array_keys(array_filter($image_style_mapping['sizes_image_styles'])),
               )
             );
             $responsive_image_style->addImageStyleMapping($breakpoint_id, $multiplier, $mapping);