Invalid Argument Error in IE

Sometime you will get Error: Invalid argument in IE. No more other information. And this script runs well in Firefox and Chrome. Indeed, this error message tells true the argument is invalid. However, the problem is not when argument being used, but lies on when you get this argument. In one case:

1 ; var horizonPosi = (window.innerWidth/2 - 50) + 'px'
2 ; headerPainting.style.left = horizonPosi

the problem actually is caused by window.innerWidth. When print out, the argument as horizonPosi is indeed "NaNpx".