Using envvars in Project Properties → Python Path does not seem to work

edit

Wing Pro 6.1.0-1 (rev 7089881dae36)

I have a couple of environment variables that I want to use as my custom Python Path. I chose Custom under Project Properties → Python Path and set its value to:

$FIRST_VAR:$SECOND_VAR

Each one of these variables has a colon-separated list of paths (all valid).  However, when I click Apply, I get an error message:

Some property warnings were encountered:
- PYTHONPATH entry 'path1:path2:path3' does not exist or cannot be found
- PYTHONPATH entry 'path4:path5' does not exist or cannot be found

What could be causing this and how do I fix it?

UPDATE:  This used to work in Wing Pro 5.x