Mask doesn't work

[Janis]

var fs = require('fs')
  , gm = require('gm');

gm(926, 517, "#ffffff")
.drawPolygon([0, 0],[100, 100],[0, 100])
.write("mask.png", function (err) {
  console.log(err);

  gm("wom.jpg").composite("wom.jpg", "mask.png")
    .write("result.png", function (err) {
    console.log(err);
    });
});

It just has zerro effect on the composite