Here's a patch against 3.5. This patch is atop the fix for #17898, so it won't apply cleanly to a stock 3.5.

The patch uses warnings.warn() to issue a warning if the plural header is empty. There seems to be precedent for issuing warnings in the case of odd incoming data or behaviour, so I'm comfortable doing this; I think that warnings need not be only for Python language features, like deprecated things.

But this does mean that importing gettext will also import warnings as a result, increasing memory usage.  Probably this doesn't matter -- on any sizable program, you probably already have the warnings module imported.  Otherwise we could print to sys.stderr.