diff -u b/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
--- b/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -300,7 +300,7 @@
     $controlling_img_attributes = $variables['attributes'];
     foreach (array('alt', 'title') as $key) {
       if (isset($variables[$key])) {
-        $controlling_img_attributes["#$key"] = $variables[$key];
+        $controlling_img_attributes[$key] = $variables[$key];
       }
     }
     $output[] = '<img' . new Attribute($controlling_img_attributes) . ' />';