Which of the following is not a legal ground for divorce?

If either pay is mentally unsound from the time of marriage the marriage is void and hence no divorce is possible. Divorce is only applicable for the dissolution of a previously valid marriage. Grounds for divorce also includes: Cruelty, deseion, mental unsoundness developing after marriage, communicable venereal diseases, renunciation, mutual consent and husband found guilty of rape, sodomy or bestiality. Ref: Textbook of Forensic Medicine and Toxicology By V.V Pillay, 15th Edition, Page 317