Fixed #1908 by testing to make sure it isn't null before checking the nodeType.

src/core.js

@@ -549,11 +549,11 @@ jQuery.extend = jQuery.fn.extend = function() {
 					continue;
 
 				// Recurse if we're merging object values
-				if ( deep && typeof options[ name ] == "object" && target[ name ] && !options[ name ].nodeType )
+				if ( deep && options[ name ] && typeof options[ name ] == "object" && target[ name ] && !options[ name ].nodeType )
 					target[ name ] = jQuery.extend( target[ name ], options[ name ] );
 
 				// Don't bring in undefined values
-				else if ( options[ name ] != undefined )
+				else if ( options[ name ] !== undefined )
 					target[ name ] = options[ name ];
 
 			}

test/unit/core.js

@@ -837,7 +837,7 @@ test("is(String)", function() {
 });
 
 test("$.extend(Object, Object)", function() {
-	expect(14);
+	expect(15);
 
 	var settings = { xnumber1: 5, xnumber2: 7, xstring1: "peter", xstring2: "pan" },
 		options =     { xnumber2: 1, xstring2: "x", xxx: "newstring" },
@@ -873,6 +873,9 @@ test("$.extend(Object, Object)", function() {
 	var ret = jQuery.extend(true, { foo: "1,2,3" }, { foo: [1, 2, 3] } );
 	ok( typeof ret.foo != "string", "Check to make sure values equal with coersion (but not actually equal) overwrite correctly" );
 
+	var ret = jQuery.extend(true, { foo:"bar" }, { foo:null } );
+	equals( ret.foo, null, "Check to make sure values equal with coersion (but not actually equal) overwrite correctly" );
+
 	var defaults = { xnumber1: 5, xnumber2: 7, xstring1: "peter", xstring2: "pan" },
 		defaultsCopy = { xnumber1: 5, xnumber2: 7, xstring1: "peter", xstring2: "pan" },
 		options1 =     { xnumber2: 1, xstring2: "x" },