Gnus: Selecting several attachments (in dired-mode) does not (seem to) work

[Marius Hofert]

Hi,

I have 
(add-hook 'dired-mode-hook 'turn-on-gnus-dired-mode)
in .gnus.el in order to use dired mode to select multiple attachments at once. 
I read that C-c C-m C-a allows this. But if I hit that key combination in a new 
message, I obtain (in the minibuffer):
C-c RET C-a is undefined

What else do I have to specify?

Cheers,

Marius