A defendant, a Hispanic 22-year-old man, was persuaded by his public defender to plead guilty to possession of marijuana to avoid a greater offense of possession with intent to distribute, even though the defendant believed that the police officer stopped and frisked him in violation of the Fourth Amendment. He pled guilty while protesting that he was innocent. The judge accepted the plea after determining that it was voluntary and intelligent. Later on, the defendant wished to challenge the constitutionally of his sentence. What is the best ground to challenge the legality of his sentence?