diff --git a/src/traversing.js b/src/traversing.js
index fb5946bb..e0f40151 100644
--- a/src/traversing.js
+++ b/src/traversing.js
@@ -73,9 +73,9 @@ jQuery.fn.extend({
 	},
 
 	is: function( selector ) {
-		return !!selector && (typeof selector === "string" ?
+		return !!selector && ( typeof selector === "string" ?
 			jQuery.filter( selector, this ).length > 0 :
-			this.filter( selector ).length > 0);
+			this.filter( selector ).length > 0 );
 	},
 
 	closest: function( selectors, context ) {
@@ -298,13 +298,18 @@ jQuery.extend({
 
 // Implement the identical functionality for filter and not
 function winnow( elements, qualifier, keep ) {
+
+	// Can't pass null or undefined to indexOf in Firefox 4
+	// Set to 0 to skip string check
+	qualifier = qualifier || 0;
+
 	if ( jQuery.isFunction( qualifier ) ) {
 		return jQuery.grep(elements, function( elem, i ) {
 			var retVal = !!qualifier.call( elem, i, elem );
 			return retVal === keep;
 		});
 
-	} else if ( qualifier && qualifier.nodeType ) {
+	} else if ( qualifier.nodeType ) {
 		return jQuery.grep(elements, function( elem, i ) {
 			return (elem === qualifier) === keep;
 		});
diff --git a/test/unit/traversing.js b/test/unit/traversing.js
index f622082e..140b337a 100644
--- a/test/unit/traversing.js
+++ b/test/unit/traversing.js
@@ -72,7 +72,7 @@ test("is(String|undefined)", function() {
 });
 
 test("is(jQuery)", function() {
-	expect(24);
+	expect(23);
 	ok( jQuery('#form').is( jQuery('form') ), 'Check for element: A form is a form' );
 	ok( !jQuery('#form').is( jQuery('div') ), 'Check for element: A form is not a div' );
 	ok( jQuery('#mark').is( jQuery('.blog') ), 'Check for class: Expected class "blog"' );
@@ -83,7 +83,6 @@ test("is(jQuery)", function() {
 	ok( !jQuery('#en').is( jQuery('[lang="de"]') ), 'Check for attribute: Expected attribute lang to be "en", not "de"' );
 	ok( jQuery('#text1').is( jQuery('[type="text"]') ), 'Check for attribute: Expected attribute type to be "text"' );
 	ok( !jQuery('#text1').is( jQuery('[type="radio"]') ), 'Check for attribute: Expected attribute type to be "text", not "radio"' );
-	ok( jQuery('#text2').is( jQuery(':disabled') ), 'Check for pseudoclass: Expected to be disabled' );
 	ok( !jQuery('#text1').is( jQuery(':disabled') ), 'Check for pseudoclass: Expected not disabled' );
 	ok( jQuery('#radio2').is( jQuery(':checked') ), 'Check for pseudoclass: Expected to be checked' );
 	ok( !jQuery('#radio1').is( jQuery(':checked') ), 'Check for pseudoclass: Expected not checked' );