encoding=utf-8 breaks my alt key on Windows

Hi,

I just installed vim-sensible, a plugin that sets a couple sane
defaults, and I found out that it broke my mappings which uses the alt
key. After a little bit of debugging I found out that setting
encoding=utf-8 caused this.

Does anyone know why this happens? And more importantly, how I can fix
this, without disabling the affecting line.

Steps to reproduce:
map <a-a> :w<cr>
set encoding=utf-8

Thanks,
Ties

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