Why is this VimLeave autocommand causing 'E749: empty buffer' ?

I'm trying to setup vim to save all files when it receives SIGTERM, here's the relevant vim command:

au VimLeave * if v:dying | wa | endif

This should write all buffers when vim exits. From another terminal if I kill vim with a modified buffer I get the following error:

Vim: Caught deadly signal TERM
Vim: preserving files...
Error detected while processing VimLeave Auto commands for "*":Vim: Finished.

E749: empty buffer

If I kill vim without modifying any buffers it doesn't report any error.

Does anyone knows why this is happening or how to fix it(or another workaround for my problem: saving all files when vim receives SIGTERM)

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