Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 30840)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -1091,7 +1091,7 @@
 			event.preventDefault();
 
 			this.params.attachment = this.params.defaultAttachment;
-			this.setting( this.params.defaultAttachment.url );
+			this.setting( '' );
  		},
 
 		/**