Bootstrap fails on windows with KeyError: 'PROGRAMFILES(X86)'

$ ./cerbero-uninstalled -c config/win32.cbc bootstrap
WARNING: Using default configuration because C:/MinGW/msys/1.0/home/Tim/.cerbero/cerbero.cbc is missing
Traceback (most recent call last):
  File ".\cerbero\utils\__init__.py", line 358, in parse_file
    exec(compile(open(filename).read(), filename, 'exec'), dict)
  File "C:/MinGW/msys/1.0/home/Tim/cerbero/config/windows.config", line 95, in <module>
    from cerbero.ide.vs.env import get_msvc_env, append_path
  File ".\cerbero\ide\vs\env.py", line 33, in <module>
    program_files = Path(os.environ['PROGRAMFILES(X86)'])
  File "c:\Python35\lib\os.py", line 683, in __getitem__
    raise KeyError(key) from None
KeyError: 'PROGRAMFILES(X86)'
Configuration Error: Could not include config file (C:/MinGW/msys/1.0/home/Tim/cerbero/config/windows.config)

This is on a 32-bit machine/install. There seems to be only PROGRAMFILES on this box:

$ printenv | grep PROG
PROGRAMDATA=C:\ProgramData
COMMONPROGRAMFILES=C:\Program Files\Common Files
PROGRAMFILES=C:\Program Files