Index: wp-includes/js/customize-preview.js
===================================================================
--- wp-includes/js/customize-preview.js	(revision 39484)
+++ wp-includes/js/customize-preview.js	(working copy)
@@ -49,7 +49,7 @@
 		history.replaceState = ( function( nativeReplaceState ) {
 			return function historyReplaceState( data, title, url ) {
 				currentHistoryState = data;
-				return nativeReplaceState.call( history, data, title, injectUrlWithState( url ) );
+				return nativeReplaceState.call( history, data, title, 'string' === typeof url && url.length > 0 ? injectUrlWithState( url ) : null );
 			};
 		} )( history.replaceState );
 
@@ -56,7 +56,7 @@
 		history.pushState = ( function( nativePushState ) {
 			return function historyPushState( data, title, url ) {
 				currentHistoryState = data;
-				return nativePushState.call( history, data, title, injectUrlWithState( url ) );
+				return nativePushState.call( history, data, title, 'string' === typeof url && url.length > 0 ? injectUrlWithState( url ) : null );
 			};
 		} )( history.pushState );