diff --git a/src/core.js b/src/core.js
index 93fe25568..5330a894b 100644
--- a/src/core.js
+++ b/src/core.js
@@ -1019,6 +1019,7 @@ $.extend( $.validator, {
 		// meta-characters that should be escaped in order to be used with JQuery
 		// as a literal part of a name/id or any selector.
 		escapeCssMeta: function( string ) {
+			if (string === undefined) return "";
 			return string.replace( /([\\!"#$%&'()*+,./:;<=>?@\[\]^`{|}~])/g, "\\$1" );
 		},