In the latest revision, add() was optimized to check for String arguments (selector.constructor == String). This breaks compability with existing code that relies on the 1.2.1 (I guess since 1.0) behaviour to silently ignore when undefined is passed as an argument.

The validation plugin relies on the old behaviour - if the new one intended behaviour, I need to release an update that handles the change before 1.2.2 gets out. Please give me a note in that case.

Testcase:

Index: test/unit/core.js
===================================================================
--- test/unit/core.js	(revision 4012)
+++ test/unit/core.js	(working copy)
@@ -183,8 +183,8 @@
 	ok( $("p").get(0) == document.getElementById("firstp"), "Get A Single Element" );
 });
 
-test("add(String|Element|Array)", function() {
-	expect(7);
+test("add(String|Element|Array|undefined)", function() {
+	expect(8);
 	isSet( $("#sndp").add("#en").add("#sap").get(), q("sndp", "en", "sap"), "Check elements from document" );
 	isSet( $("#sndp").add( $("#en")[0] ).add( $("#sap") ).get(), q("sndp", "en", "sap"), "Check elements from document" );
 	ok( $([]).add($("#form")[0].elements).length >= 13, "Check elements from array" );
@@ -196,6 +196,9 @@
 	var x = $([]).add("<p id='x1'>xxx</p>").add("<p id='x2'>xxx</p>");
 	ok( x[0].id == "x1", "Check on-the-fly element1" );
 	ok( x[1].id == "x2", "Check on-the-fly element2" );
+	
+	var notDefined;
+	equals( $([]).add(notDefined).length, 0, "Check that undefined adds nothing." );
 });
 
 test("each(Function)", function() {

Patch:

Index: src/core.js
===================================================================
--- src/core.js	(revision 4012)
+++ src/core.js	(working copy)
@@ -340,6 +340,8 @@
 	},
 
 	add: function( selector ) {
+		if (!selector)
+			return this;
 		return this.pushStack( jQuery.merge( 
 			this.get(),
 			selector.constructor == String ?