Imac 27 (late 2009) doesn't work as external display

Hi,

I've just bought a brand new macbook pro 13 Retina and try to use my iMac 27 (late-2009) as a secondary display.

It doesn't work...

I connect them using a thunderbolt cable but iMac is not recognized at all even if I try to force the target mode (Command + F2).

I red the forum and the following article but nothing works .

http://support.apple.com/kb/HT3924

Any idea ?

Thanks,

Olivier