Piston Does Not Extend with a Slime Block in front of it.

What I expected to happen was...

The Piston would extend, pushing the slime block with it.

What happened instead was...
The Piston does nothing.

Steps To Reproduce

Step 1: Create a World with the 14w18b snapshot [On Creative]
Step 2: Place a slime block, with a piston behind it.
Step 3: Apply a redstone signal to the piston.