When running ./mach xpcshell-test --headless netwerk/cookie/test/browser/browser_indexedDB.js, I get the following error:

Error running mach:

    ['xpcshell-test', '--headless', 'netwerk/cookie/test/browser/browser_indexedDB.js']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file xpcshell-test| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

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

The details of the failure are as follows:

TypeError: can only concatenate str (not "bytes") to str

  File "/home/simon/work/mozilla-unified/testing/xpcshell/mach_commands.py", line 257, in run_xpcshell_test
    return xpcshell.run_test(**params)
  File "/home/simon/work/mozilla-unified/testing/xpcshell/mach_commands.py", line 57, in run_test
    return self.run_suite(**kwargs)
  File "/home/simon/work/mozilla-unified/testing/xpcshell/mach_commands.py", line 43, in run_suite
    return self._run_xpcshell_harness(**kwargs)
  File "/home/simon/work/mozilla-unified/testing/xpcshell/mach_commands.py", line 130, in _run_xpcshell_harness
    result = xpcshell.runTests(kwargs)
  File "/home/simon/work/mozilla-unified/testing/xpcshell/runxpcshelltests.py", line 1553, in runTests
    self.buildEnvironment()
  File "/home/simon/work/mozilla-unified/testing/xpcshell/runxpcshelltests.py", line 1152, in buildEnvironment
    "llvm-symbolizer" + self.mozInfo["bin_suffix"].encode('ascii'))

https://hg.mozilla.org/mozilla-central/rev/f8542981907c