signs on fences bug

I was playing on my server which has updated to 1.6.
I tried to place a sign on a fence whilst in creative mode but no gui to let me type appeared. it just placed a blank sign on the side of the fence. this happened with both normal and nether brick fences. signs seem to work on other blocks (I haven't tried all blocks though) but not on fences... it does the same thing in survival mode too.