$ ./mach test security/manager/ssl/tests/unit/test_broken_fips.js
 e:/m/mozilla-unified/obj-x86_64-pc-mingw32/backend.TestManifestBackend is out of date with respect to e:/m/mozilla-unified/python/mozbuild/mozbuild/frontend/gyp_reader.py
Test configuration changed. Regenerating backend.
 0:02.57 INFO Found node at c:\Users\kimu\.mozbuild\node\node.EXE
 0:02.57 INFO Found moz-http2 at e:\m\mozilla-unified\testing\xpcshell\moz-http2\moz-http2.js
Error running mach:

    ['test', 'security/manager/ssl/tests/unit/test_broken_fips.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| 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: environment can only contain strings

  File "e:\m\mozilla-unified\testing/mach_commands.py", line 387, in test
    argv=extra_args, test_objects=tests, **kwargs)
  File "e:\m\mozilla-unified\python/mach\mach\registrar.py", line 152, in dispatch
    return self._run_command_handler(handler, context=context, **kwargs)
  File "e:\m\mozilla-unified\python/mach\mach\registrar.py", line 109, in _run_command_handler
    result = fn(**kwargs)
  File "e:\m\mozilla-unified\testing/xpcshell/mach_commands.py", line 261, in run_xpcshell_test
    return xpcshell.run_test(**params)
  File "e:\m\mozilla-unified\testing/xpcshell/mach_commands.py", line 60, in run_test
    return self.run_suite(**kwargs)
  File "e:\m\mozilla-unified\testing/xpcshell/mach_commands.py", line 46, in run_suite
    return self._run_xpcshell_harness(**kwargs)
  File "e:\m\mozilla-unified\testing/xpcshell/mach_commands.py", line 136, in _run_xpcshell_harness
    result = xpcshell.runTests(filtered_args)
  File "e:\m\mozilla-unified\testing/xpcshell\runxpcshelltests.py", line 1357, in runTests
    self.trySetupNode()
  File "e:\m\mozilla-unified\testing/xpcshell\runxpcshelltests.py", line 1162, in trySetupNode
    startServer('moz-http2', os.path.join(myDir, 'moz-http2', 'moz-http2.js'))
  File "e:\m\mozilla-unified\testing/xpcshell\runxpcshelltests.py", line 1144, in startServer
    stderr=PIPE, env=self.env, cwd=os.getcwd())
  File "d:\mozilla-build\python\lib\subprocess.py", line 394, in __init__
    errread, errwrite)
  File "d:\mozilla-build\python\lib\subprocess.py", line 644, in _execute_child
    startupinfo)

$

https://hg.mozilla.org/mozilla-central/rev/cf84cc29a5f2