Quote fromAn error that an opponent has no opportunity to verify the legality of should have its penalty upgraded. These errors involve misplaying hidden information, such as the morph ability or failing to reveal a card to prove that a choice made was a legal one. If the information was ever in a position where opponents had the opportunity to verify the legality (such as on top of the library, as the only card in hand, or on the battlefield), do not upgrade the penalty and reveal the information if possible.
Since the card was on top of the library before it was drawn, it was in a position where the opponent could verify the information (in this case, by pointing out that it should be revealed). The solution is to reveal the card if possible (in addition to the warning).