5 month male marking inside house

by Katie
(NJ)

I have a five month old Pomeranian male who for the most part is very good natured and willing to listen. He mostly goes outside, rarely has an accidents, doesn't chew on my wooden furniture, and has been successfully taught not to bite any one anymore.

However when he is not allowed in a room, and the door is shut on him as a result, he marks the door. From what I could figure out it seems to be out of anger.

I have no idea how to handle this situation. I have researched but have not found any straight answers as of yet. So I'm wondering if anyone has advice :)