Index: customize/class-wp-customize-background-image-setting.php
===================================================================
--- customize/class-wp-customize-background-image-setting.php	(revision 53902)
+++ customize/class-wp-customize-background-image-setting.php	(working copy)
@@ -26,10 +26,8 @@
 
 	/**
 	 * @since 3.4.0
-	 *
-	 * @param mixed $value The value to update. Not used.
 	 */
-	public function update( $value ) {
+	public function update() {
 		remove_theme_mod( 'background_image_thumb' );
 	}
 }