Prostitute Jailed for Robbery

From the headline in this story I thought the crime might be based on fraud--a prostitute who took money and refused to perform services.  In fact, it was a traditional forcible robbery.   But I would think that because the contract for sexual services is illegal, a prostitute's taking of money, even with no intent to perform any services, would not be criminal.  Or is the fact that there might be a contract defense to a civil suit irrelevant to the question of criminal liability for fraud or false pretenses? [Jack Chin]