Can't write on signs placed on fences

In survival or creative, you cannot type on a sign placed on a fence

(tested on superflat(overworld mode))

What I expected to happen was...:
I place a sign on a fence and write on the sign

What actually happened was...:
The sign was placed and the text box did not come up

Steps to Reproduce:
1. Put a sign on top, or on the side of a fence
2. Sign is placed
3. Nothing