tmux disregarding the configuration file

I just installed tmux on Ubuntu 10.04 and tried to remap the prefix key to C-a by creating the file ~/.tmux.conf with these lines:

set-option -g prefix C-aunbind-key C-b

When starting tmux, both regularly and with -f ~/.tmux.conf, the prefix is effectively the same default C-b

There are no errors or warnings whatsoever.

Any idea what's wrong with the loading of the conf?