signs can't be typed on when placed on top of a fence post (not confirmed for nether brick fences)

What I expected to happen was...:
placed a sign on top of a fence post. was going to type on it.

What actually happened was...:
it only placed the sign, didn't even open the typing gui

Steps to Reproduce:
1. place a sign on a fence post
2. be ready for absolutely nothing
3. ...