Comment 18 for bug 1528139

Hmm, this looks like you don't have the latest trunk checkout - this is bug 1525119 which is fixed since 2015-12-12.

Another detail confirms that you have an outdated checkout:
    rmask = rmask.replace('c', 'a')
We changed 'a' to 'w' a month ago ;-) (see bzr log -r3279 for background info)