AS3 - mask = null problem

programming flash
1

Hi, I have a movieclip which is being masked using another movieclip. In one condition I want to unmask the mc. But seems mask = null isn’t working. Any idea how can I remove mask dynamically.

a.mask = b;
a.width ++;

if(a.width > 10)
{
a.mask = null; //nothing happens :frowning:
}