diff -u b/core/modules/responsive_image/src/Entity/ResponsiveImageStyle.php b/core/modules/responsive_image/src/Entity/ResponsiveImageStyle.php
--- b/core/modules/responsive_image/src/Entity/ResponsiveImageStyle.php
+++ b/core/modules/responsive_image/src/Entity/ResponsiveImageStyle.php
@@ -186,9 +186,11 @@
       $this->addDependency($type, $provider);
     }
     foreach ($this->getImageStyleMappings() as $mapping) {
-      $style = ImageStyle::load($mapping['image_style']);
-      if ($style) {
-        $this->addDependency('config', $style->getConfigDependencyName());
+      if (isset($mapping['image_style'])) {
+        $style = ImageStyle::load($mapping['image_style']);
+        if ($style) {
+          $this->addDependency('config', $style->getConfigDependencyName());
+        }
       }
     }
     return $this->dependencies;