WHAT THE HELL DOES “This housing is suitable.” MEAN!??!

I have built a series of houses for my NPCs to live in. However, there is one very, very lonely house. When I try to assign my NPCs to that particular house, I get a message that says “This housing is suitable.” OK, so lets have someone live there, shall we? Oh, I can’t? Why? You said yourself that the place is suitable. So what are we getting hung up on here? This text obviously comes up when something is wrong with the place I’m trying to put someone, but it is so non-descriptive it might as well say “No”. Please address this issue in future updates.