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Some loyalty cards let you rack up points, but one business in Malaysia allegedly offered its customers something they could really use: free sex after nine car washes.

Police say they busted the car wash near Kuala Lumpur last week after discovering it had a partnership with a massage parlour, the Malay Mail reports.

They were tipped off when they raided the parlour and found the car wash cards on several alleged customers of prostitutes there.

"It was supposed to be just another routine operation," Emmi Shah Fadhil, the officer in charge, told the paper.

"To get the extra 'offer' customers must send their cars for washing nine times within a certain period. The tenth car wash will entitle them to free sex," Fadhil said, which would usually cost between 130 and 180 Malaysian ringgit (about $40-56).

Nine Vietnamese women, aged 18 to 28, were arrested on suspicion of prostitution. Four men suspected of running the scheme were also detained.