Pistons do not push lead connected to fences

Title says it all.
Whenever I try to push any mob connected to a lead via a fence, the fence will be pushed away yet the lead will hang there in midair until it's out of reach of the fence. As a result the lead will obviously break and the mob will fall to it's death

I recorded a video giving it a clearer view of this situation

PS: sorry for the bad quality, but it was the only way I could upload it without exceeding the maximum file size