Why can't I have two cohorts?

I know, this is a late time to be posting about a game that most people probably stopped playing 2 years ago but I just got it last week. I sunk a feat slot on my cleric, thinking hey this would be a pretty cool feat for my charismatic leader type drow cleric.

So, I get the feat at level 6, having the ranger in my party and the druid parked in the tavern. I go to the tavern, click on the registration book, except when I try to add the druid it acts like I don't have any space available even when the screen says I have 1 spot left.

I'm not using any cheats or mods in my game, and I'm patched up to the latest version.