Can't get imap to work in Vim 7

[Sean Lerner]

Hello,

I'd like to toggle line number on and off in Vim 7. I've set:
map <F11> :set invnumber <CR>
imap <F11> <ESC>:set invnumber <CR>li

And I am able to toggle line numbers when in command mode, but not when 
in insert mode.

When I do press F11 on my keyboard in insert mode, it just types out <F11>.

I've tested these settings on a different server running Vim 6.3.71 and 
they work okay.

I've tried mapping other things using imap in Vim 7, but they don't work 
either.

Any thoughts?

thanks,

Sean