Can't place gate on fence??

so i went to go build a prison using the fence & gate from the Get To Work expansion, the one with the barbed wire on the top.. But the gate wont go on the fence even though they are the same? It just keeps saying "must be placed on a shorter fence" so i tried that too, and it said the same thing.. Not really sure whats going on with that. I restarted my game and still doesn't work. It's the same for the other fence that came with the same expansion, the one used in the Police Station for the cells??