Canvas ImageData can't save values as 0

On changing canvas imagedata object it cannot set values as 0.
Setting range 1-255 works normally.

Steps to reproduce:
1. Create Canvas element and get 2d context
2. Create imageData e.g. 1x1 size
3. Set first element of data (red channel) and fourth element (alpha) as 255
4. Set first element as 0
5. Console this element

As you can see, our value is still 255.

The same canvas code works in browser (tested on Chromium 22.0 - see screenshot).

For easiest debug I attach results and code for 100x100 canvas size.