Google App Engine for Business error with appcfg.py

I have "App Engine for Business" access at work, which lets me create internal-only App Engine apps.

But upon creating a new internal app, I kept getting "Error 404" messages about "This application does not exist" when I tried to use appcfg update to upload it.

Turns out there's a special syntax for the application name in the app.yaml file. I changed the "application: myapp" line to "application: mycompany.com:myapp" and then it worked.