diff --git a/src/css/adjustCSS.js b/src/css/adjustCSS.js
index 626ec7475..8898789a7 100644
--- a/src/css/adjustCSS.js
+++ b/src/css/adjustCSS.js
@@ -19,7 +19,8 @@ function adjustCSS( elem, prop, valueParts, tween ) {
 		unit = valueParts && valueParts[ 3 ] || ( jQuery.cssNumber[ prop ] ? "" : "px" ),
 
 		// Starting value computation is required for potential unit mismatches
-		initialInUnit = ( jQuery.cssNumber[ prop ] || unit !== "px" && +initial ) &&
+		initialInUnit = elem.nodeType &&
+			( jQuery.cssNumber[ prop ] || unit !== "px" && +initial ) &&
 			rcssNum.exec( jQuery.css( elem, prop ) );
 
 	if ( initialInUnit && initialInUnit[ 3 ] !== unit ) {
diff --git a/test/unit/effects.js b/test/unit/effects.js
index 462524ef3..e297273e4 100644
--- a/test/unit/effects.js
+++ b/test/unit/effects.js
@@ -607,6 +607,17 @@ QUnit.test( "animate non-element", function( assert ) {
 	this.clock.tick( 200 );
 } );
 
+QUnit.test( "animate non-element's zIndex without appending \"px\"", function( assert ) {
+	assert.expect( 1 );
+
+	var obj = { zIndex: 0 };
+
+	jQuery( obj ).animate( { zIndex: 200 }, 200, function() {
+		assert.equal( obj.zIndex, 200, "The custom property should be modified without appending \"px\"." );
+	} );
+	this.clock.tick( 200 );
+} );
+
 QUnit.test( "stop()", function( assert ) {
 	assert.expect( 4 );