compose: Docker-compose rejects windows style volume mappings

When building windows containers a compose file like:

foo:
    image: windowsservercore
    volumes:
        - C:\foo:C:\bar:ro

Results in the error: ERROR: Volume c:\foo:c:\bar:ro has incorrect format, should be external:internal[:mode]

But if I call docker directly with “docker run -v C:\foo:C:\bar:ro windowsservercore” it is accepted as expected.