git-send-email Warning: unable to close filehandle properly: Bad file descriptor during global destruction.

I just used git-send-email the first time since ages. When it finished,
I saw a stream of 16 warnings:

Warning: unable to close filehandle properly: Bad file descriptor during
global destruction.

The invocation was this:

   git send-email --to git@vger.kernel.org patches

where the argument 'patches' is a directory with 4 patches.

The patches were sent quite fine, as it seems. I can help debug if
someone is interested.

-- Hannes