A thief who stole a flowerpot containing the ashes of an African witch doctor grandmother is risking an involuntary sex change, her grandson warned in a letter published Thursday in a New Zealand newspaper.

The pot was stolen from the home at Gulf Harbour, near Auckland, of At du Plooy, who wrote to the Rodney Times saying he was a medium and keeps in touch with spirit of his grandmother who died 16 years ago in Africa at age 93.

She had put a curse on an area one kilometre around the pot with sex-change icons that can gradually change men into women and vice versa, unless the pot is returned, he wrote.

Du Plooy said he thought he could trace the pot with her spiritual help, but his grandma was not familiar with the area where it had been taken.