Assign custom property

1

Hi
i try to migrate from as2 to as3 and i have one problem
When i attachMovie from library (addChild in as3) and i add custom property
its work fine.
Here is ex.

var i:int;
for (i=0; i<5; i++)
{
	var link:MovieClip = new Link();
	addChild(link);
	link.x = i * 100;
	link.customPropery= "example";
	trace(link.customPropery);
}

The problem is when i attach to that symbol custom class in the library
i got the error:
#1056: Cannot create property…
What i miss here?
sorry if you already mention that in the forum…
Thanks