diff --git a/src/js/_enqueues/vendor/plupload/handlers.js b/src/js/_enqueues/vendor/plupload/handlers.js
index 37b56631cb..81d91e9f88 100644
--- a/src/js/_enqueues/vendor/plupload/handlers.js
+++ b/src/js/_enqueues/vendor/plupload/handlers.js
@@ -601,7 +601,7 @@ jQuery( document ).ready( function( $ ) {
 			if ( file.type && file.type.indexOf( 'image/' ) === 0 ) {
 				up.settings.multipart_params._wp_temp_image_ref = file.id;
 			} else {
-				unset( up.settings.multipart_params._wp_temp_image_ref );
+				up.settings.multipart_params._wp_temp_image_ref = '';
 			}
 		} );
 	};
diff --git a/src/js/_enqueues/vendor/plupload/wp-plupload.js b/src/js/_enqueues/vendor/plupload/wp-plupload.js
index 3200f316ac..7677cc66ea 100644
--- a/src/js/_enqueues/vendor/plupload/wp-plupload.js
+++ b/src/js/_enqueues/vendor/plupload/wp-plupload.js
@@ -326,7 +326,7 @@ window.wp = window.wp || {};
 			if ( file.type && file.type.indexOf( 'image/' ) === 0 ) {
 				up.settings.multipart_params._wp_temp_image_ref = file.id;
 			} else {
-				unset( up.settings.multipart_params._wp_temp_image_ref );
+				up.settings.multipart_params._wp_temp_image_ref = '';
 			}
 		} );