diff --git a/src/css/curCSS.js b/src/css/curCSS.js
index e09574530..1af1812cd 100644
--- a/src/css/curCSS.js
+++ b/src/css/curCSS.js
@@ -51,7 +51,9 @@ if ( window.getComputedStyle ) {
 			}
 		}
 
-		return ret;
+		// Support: IE
+		// IE returns zIndex value as an integer.
+		return ret === undefined ? ret : ret + "";
 	};
 } else if ( document.documentElement.currentStyle ) {
 	getStyles = function( elem ) {
@@ -99,7 +101,11 @@ if ( window.getComputedStyle ) {
 			}
 		}
 
-		return ret === "" ? "auto" : ret;
+		return ret === "" ? "auto" :
+			// Support: IE
+			// IE returns zIndex value as an integer.
+			ret === undefined ? ret :
+			ret + "";
 	};
 }
 
diff --git a/test/unit/css.js b/test/unit/css.js
index e6f9c91df..56d22bdf2 100644
--- a/test/unit/css.js
+++ b/test/unit/css.js
@@ -200,6 +200,16 @@ test( "css() explicit and relative values", 29, function() {
 	equal( $elem.css("opacity"), "1", "'+=0.5' on opacity (params)" );
 });
 
+test("css(String) where values are z-index", function() {
+	expect(1);
+
+	var $elem = jQuery( "<div>" ).appendTo( "#qunit-fixture" );
+
+	$elem.css({ "position": "absolute", "z-index": "1000" });
+	strictEqual( $elem.css( "z-index" ), "1000" );
+});
+
+
 test("css(String, Object)", function() {
 	expect( 19 );
 	var j, div, display, ret, success;
@@ -393,6 +403,14 @@ test("css(Object) where values are Functions", function() {
 	jQuery("#cssFunctionTest").remove();
 });
 
+test("css(String) where values are undefined", function() {
+	expect(1);
+
+	var $elem = jQuery( "#nothiddendiv" );
+
+	strictEqual( $elem.css( "test" ), undefined );
+});
+
 test("css(Object) where values are Functions with incoming values", function() {
 	expect(3);