Canvas DrawImage and alpha transparent does not work on IOS Safari

I try to combine two images with HTML5 canvas. iOS Safari does not honor alpha channels.

This example code works in all other modern browsers but not Safari mobile. In Safari the second image (a transparent png overlay) is not transparent.

canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
context = canvas.getContext('2d');
context.globalAlpha = 1;
context.drawImage($element.find('.resize-image')[0], left, top, width, height, 0, 0, width, height);
context.drawImage($element.find('.resize-image-overlay')[0], 0, 0);
dataUrl = canvas.toDataURL("image/png");

Does anyone have any input on this?