Format elixir on save VIM

Questions & Help Questions
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3

I added this to my .vimrc:

" format elixir files when saving
augroup filetype_ex
	autocmd!
	autocmd BufPreWrite *.ex,*.exs mix format
augroup END

But I get this error:

Error detected while processing /home/pedro/.vim/vimrc:
line  140:
E216: No such group or event: BufPreWrite *.ex,*.exs mix format

What am I missing?