something broken in wpt run

I just did a brand-new clone of web-platform-tests, and now when I try to run the tests:

Python wpt run edge weddriver

I get an error:

Traceback (most recent call last):
  File "wpt", line 5, in <module>
    wpt.main()
  File "C:\users\johnjan\documents\github\web-platform-tests\tools\wpt\wpt.py", line 111, in main
    venv = setup_virtualenv(main_args.venv, props)
  File "C:\users\johnjan\documents\github\web-platform-tests\tools\wpt\wpt.py", line 84, in setup_virtualenv
    venv.start()
  File "C:\users\johnjan\documents\github\web-platform-tests\tools\wpt\virtualenv.py", line 47, in start
    self.activate()
  File "C:\users\johnjan\documents\github\web-platform-tests\tools\wpt\virtualenv.py", line 42, in activate
    execfile(path, {"__file__": path})
IOError: [Errno 2] No such file or directory: 'C:\\users\\johnjan\\documents\\github\\web-platform-tests\\_venv\\Scripts\\activate_this.py'

Is there some new step I need to take to force the Scripts directory to get created?

This worked last night before I got the changes from this weekend.

Can you help?