diff --git a/src/renderable/container.js b/src/renderable/container.js
index <HASH>..<HASH> 100644
--- a/src/renderable/container.js
+++ b/src/renderable/container.js
@@ -112,6 +112,11 @@
 				}
             }
 
+            // specify a z property to infinity if not defined
+			if (typeof child.z === 'undefined') {
+				child.z = Infinity;
+			}
+
 			child.ancestor = this;
             
 			this.children.push(child);
diff --git a/src/vendors/tween.js b/src/vendors/tween.js
index <HASH>..<HASH> 100644
--- a/src/vendors/tween.js
+++ b/src/vendors/tween.js
@@ -48,11 +48,6 @@
 		var _onUpdateCallback = null;
 		var _onCompleteCallback = null;
 		
-		/**
-		 * fixed z value
-		 * @ignore
-		 */
-		this.z = 999;
 
 		// Set all starting values present on the target object
 		for ( var field in object ) {
removed the `z` property from the Tween object definition

this is now enforced through the `addChild` function that will set the
property to `Infinity` if not defined.

the Tween object is now almost the default one without any specific
(melonJS) properties.