This might not be the problem in this case, but this still bugs me. You’re using cast without telling Haxe what to cast it to. This can make the compiler fail to notice errors, and Haxe will use reflection to access the properties, which is slightly slower.
It can also prevent you from setting certain variables, including height. But if that was happening here, you’d see a different error message.
Even if it won’t fix your problem here, just type out getMask:MovieClip (or whatever class), and you could save yourself future headaches.