What is the ruling on a husband having intercourse with his wife while imagining she is his mother?

It is forbidden for a man to imagine another woman having intercourse with him while he is having intercourse with his wife. This is even more forbidden if the imagined woman is one of his unmarriageable kin (mahram), such as his wife's mother, because that resembles imagining intercourse with one's sister or maternal aunt. This has been considered a form of zina (fornication/adultery), as mentioned by Ibn al-Hajj al-Maliki, and Ibn Aqil stated that the doer of this is sinful.