diff --git a/wp-includes/js/customize-controls.dev.js b/wp-includes/js/customize-controls.dev.js
index 0b4baf1ccb..3439b1d73b 100644
--- a/wp-includes/js/customize-controls.dev.js
+++ b/wp-includes/js/customize-controls.dev.js
@@ -252,7 +252,7 @@
 		api('background_color').method = 'postMessage';
 
 		// api('background_image').method = 'postMessage';
-		api('background_image').uploader.param( 'post_data', { context: 'custom-background' });
+		api('background_image').uploader.param( 'post_data[context]', 'custom-background' );
 	});
 
 })( wp, jQuery );
\ No newline at end of file