If I try to run mach wpt I get an error. It was working a few days ago, but I updated my tree today and it stopped.

Error running mach:

['wpt']

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: unhashable type: 'dict'

File "/Users/andrewmccreight/mc/testing/web-platform/mach_commands.py", line 377, in run_wpt
return self.run_web_platform_tests(**params)
File "/Users/andrewmccreight/mc/testing/web-platform/mach_commands.py", line 369, in run_web_platform_tests
return wpt_runner.run(logger, **params)
File "/Users/andrewmccreight/mc/testing/web-platform/mach_commands_base.py", line 44, in run
result = wptrunner.start(**kwargs)
File "/Users/andrewmccreight/mc/testing/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 383, in start
rv = not run_tests(**kwargs) or logged_critical.has_log
File "/Users/andrewmccreight/mc/testing/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 242, in run_tests
logger.suite_start(test_source_cls.tests_by_group(tests, **test_source_kwargs),
File "/Users/andrewmccreight/mc/testing/web-platform/tests/tools/wptrunner/wptrunner/testloader.py", line 379, in tests_by_group
return {cls.group_metadata(None): [t.id for t in tests]}

It looks like this line was added by bug 1614643.

My apologies, there's a fix on autoland that should be merging soon.