Buttonmode not working!

1

I have created an array which stores all the MovieClips which I want to use as button then I am passing that array to other class using setter and trying to enable buttonmode ,but its not working ,its giving error “Cannot access a property or method of a null object reference.”;
But all other event listeners are working.
Here is my code


for(var i:int=0;i<_btnName.length;i++)
	
{
      (_btnName* as MovieClip).buttonMode=true;
       
}