Is one who approaches a sorcerer and asks him to solve a problem, while knowing the invalidity of the sorcerer's action, considered an unbeliever?

Visiting sorcerers and asking them questions, even without believing them, is considered a major sin (kabira), and the prayers of the person who does so will not be accepted for forty nights. As for believing what they say, it is considered disbelief (kufr) in what was revealed to Muhammad, because soothsaying (kahana) includes every claim to knowledge of the unseen.