Index: wp-includes/js/customize-preview.js
===================================================================
--- wp-includes/js/customize-preview.js	(revision 39483)
+++ wp-includes/js/customize-preview.js	(working copy)
@@ -31,7 +31,7 @@
 		injectUrlWithState = function( url ) {
 			var urlParser, oldQueryParams, newQueryParams;
 			urlParser = document.createElement( 'a' );
-			urlParser.href = url;
+			urlParser.href = url || '';
 			oldQueryParams = api.utils.parseQueryString( location.search.substr( 1 ) );
 			newQueryParams = api.utils.parseQueryString( urlParser.search.substr( 1 ) );