Index: src/wp-includes/js/heartbeat.js
===================================================================
--- src/wp-includes/js/heartbeat.js	(revision 25023)
+++ src/wp-includes/js/heartbeat.js	(working copy)
@@ -427,7 +427,7 @@
 		 */
 		this.enqueue = function( handle, data, dont_overwrite ) {
 			if ( handle ) {
-				if ( queue.hasOwnProperty( handle ) && dont_overwrite )
+				if ( dont_overwrite && queue.isQueued( handle ) )
 					return false;
 
 				queue[handle] = data;
@@ -443,6 +443,9 @@
 		 * $return mixed The data queued with that handle or null
 		 */
 		this.isQueued = function( handle ) {
+			if (! queue.hasOwnProperty( handle ))
+				return null;
+
 			return queue[handle];
 		};
 	};