GetItemSpell, when can I call it ?

[Sha2700]

Hello!

I started writing an addOn for fun, and I begin to have some timing issues...
On event "PLAYER_ENTERING_WORLD", I create the full addOn interface, registering events, etc...
Everything is OK except on login where a GetItemSpell and returns nil... After that, no more issues.
Is there any timing constraints for this function ? Does the event "PLAYER_ENTERING_WORLD" happen to soon and the Blizzard's internals are not yet fully loaded ?

Thanks !