Guard cannot target another guard?

[Moondy]

This happened to me, even I was sure he was a guide "guard" was not a listed option.
Is there a rule about this or is it a bug?

[Jellby]

The rules summary says "choose a player and name a card value (other than 1)", emphasis mine. I'm sure the official rules say the same or something similar.

[ufm]

Guards cannot target other Guards. There is even an in-game explanation for that (in Japanese version, at least).

[Earthboundia]

From a gameplay mechanic point of view, you are significantly more likely to eliminate a player if you guess guard than any other number since there are at least double the number of guards more than any other number, in any version of the game. From the in game lore point of view, by playing a guard you are accusing your fellow guard of having a love letter to the princess (or perhaps even accusing yourself since I'm fairly certain all the guards are technically the same person).