diff --git a/ui/ui.core.js b/ui/ui.core.js
index 12ee73737..ddeb7d5e2 100644
--- a/ui/ui.core.js
+++ b/ui/ui.core.js
@@ -423,6 +423,8 @@ $.ui.mouse = {
 
 	_mouseDown: function(event) {
 		// don't let more than one widget handle mouseStart
+		// TODO: figure out why we have to use originalEvent
+		event.originalEvent = event.originalEvent || {};
 		if (event.originalEvent.mouseHandled) { return; }
 
 		// we may have missed mouseup (out of window)