Doom 3 keeps telling me cd key is wrong

[feest]

Hi, i've installed doom3 and when i'm in the main screen it asks for the cd key, okay i grab it and enter it. however, it keeps telling me my serial is not correct. whats wrong?

[Mapassy]

u must be offline

[feest]

offline?

[RomeReactor]

You must be online for the game to validate your cd-key.