Servo python build error Windows 10

Compiling Servo on Windows 10 I hit the following error

./mach build -r
<time passes>
--- stderr
thread '<main>' panicked at 'Can't find python (tried python27.exe and python.exe)! Try fixing PATH or setting the PYTHON env var', C:\msys64\home\kbrosnan\src\servo\components\style\
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Setting the python environment variable allowed the build to succeed. export PYTHON=/c/Python27/python.exe