Is it permissible to pray behind an imam who practices gambling (cards)?

The prayer behind the aforementioned Imam is valid, because whoever's prayer is valid for himself, it is valid for others. However, it is preferable to pray behind someone else who is free from committing a sharia violation. The said Imam should be advised, and the gravity of his gambling, which Allah Almighty said about, should be explained to him: (They ask you about wine and gambling. Say, "In them is great sin and [yet, some] benefits for people. But their sin is greater than their benefit.")