AS3 function running before variables are defined!

Question

I am trying to add an init() function to a MovieClip, but when I run the function from scene1 the variables that were set in the MovieClip are not defined yet... The MovieClip was dragged to the stage from the library.

scene1:

mc.init(null);

MovieClip:

var _default = 5;

function init(num) {
     if(num == null) {
          trace(_default);
     } else {
          trace(num);
     }
}

This is tracing "undefined" instead of "5"; Is there a way of fixing this problem?