diff --git a/src/core.js b/src/core.js
index 0af71985c..c69898534 100644
--- a/src/core.js
+++ b/src/core.js
@@ -1320,7 +1320,7 @@ $.extend( $.validator, {
 
 		// Evaluate parameters
 		$.each( rules, function( rule, parameter ) {
-			rules[ rule ] = $.isFunction( parameter ) && rule !== "normalizer" ? parameter( element ) : parameter;
+			rules[ rule ] = typeof parameter === "function" && rule !== "normalizer" ? parameter( element ) : parameter;
 		} );
 
 		// Clean number parameters