In Genesis 4:7, what was Qayin (Cain) told to rule over or master?

Answer From The Annual Reading Schedule - Week 1 Notes For Genesis: The door. The door? Yes – the door. The Hebrew says "you will rule/master him". There is no "it" in Hebrew as some translations say "master it". Many believe this is saying to master sin, but the word for sin in Genesis 4:7 is Chaththat (or Chattat) which is a feminine noun according to Brown-Driver-Briggs' Hebrew Definitions. The word for door in Genesis 4:7 is Petach which is a masculine noun that makes sense also if we understand it properly. Jeff Benner's interpretation in his 2008 Genesis lessons online that the "him" is referring to Hevel (Abel) and that Yahweh is teaching Qayin (Cain) that he should set a good example for his brother (i.e. "master him") because Qayin is the older brother. However, I point out that the word door is in this verse and is masculine so it makes more sense that it is referring to the door. The door to what? The door to Qayin's heart. It seems that Yahweh is teaching that Qayin should guard the door to His heart – to guard His heart – which lines up with other Scriptures such as Proverbs 4:23.