[ramdac@ramdac mozilla-central]$ ./mach robocop testSelectionHandler
Error running mach:

    ['robocop', 'testSelectionHandler']

The error occurred in the implementation of the invoked mach command.

This should never occur and is likely a bug in the implementation of that
command. Consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

TypeError: coercing to Unicode: need string or buffer, NoneType found

  File "/home/ramdac/Projects/mozilla-central/testing/mochitest/mach_commands.py", line 907, in run_robocop
    '--xre-path=' + os.environ.get('MOZ_HOST_BIN'),