ActionScript 3.0 :: Warning: 1098: Illogical Comparison With NaN

I have a puzzle game, in which the user can define how many pieces will the puzzle have. There are some problems in the entry validation. I pass the data in a custom event, and then check if they are numbers, if they're not, then go with the default values.
I'm getting this warning: Warning: 1098: Illogical comparison with NaN. This statement always evaluates to false.
It turns out the warning is right, even despite the traces, which (when I input letters instead of numbers), show both variables as NaN.

Code:
private function initGame(e:MenuEvent):void
{
game = new Game();
game.addEventListener(GameEvent.OVER, onGameOver, false, 0, true);

[code]....