Vim mapping works in command mode but not when used in autocmd

Question

I have this autocmd in my .vimrc:

autocmd Filetype markdown inoremap <buffer> <CR> <C-o>:call AutoUMDList()<CR>

When I open a markdown file, enter Insert mode and press <CR>, I get the following error:

E15: Invalid expression: ^O:call AutoUMDList()^M
E15: Invalid expression: ^O:call AutoUMDList()^M
Press ENTER or type command to continue

But if I go in Insert mode and type <C-o>:call AutoUMDList()<CR> it works!

Also, if I type the following in Command mode it also works:

:inoremap <buffer> <CR> <C-o>:call AutoUMDList()<CR>

What is going on here please?! I don't understand why my autocmd won't work...