[RMMV] HELP WITH SCRIPT

Hello, i want to draw a bigger icon (4 times) on a window. I have edited the 'IconSet' file and i have written this code:

var iconIndex = item.iconIndex;
var bitmap = ImageManager.loadSystem('BigIconSet');
var pw = 128;
var ph = 128;
var sx = iconIndex % 16 * pw;
var sy = Math.floor(iconIndex / 16) * ph;
this.contents.blt(bitmap, sx, sy, pw, ph, 0, 0);

But just appear the empty window, someone can help me, please?