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I guess it must be from the August Humble Monthly Bundle, am I right?
If so, could you check your Humble library? It should list the countries where it cannot be redeemed.

95% sure this is the lock in question. The other locks are for the Middle East, Latin America, RU/CIS and Japan. I take it you are from one of those regions?

Usually RoW keys are fine, but Activision felt the need to block Non-RoW peeps from activating RoW copies of the game. No clue as to why they would do that since RoW copies are generally more expensive but w/e.

When in doubt, you can always check the packages on steamdb.info for more information.