Piston activating when it should not activate.

I placed redstone with no signal in-front of a piston with a slimeblock and a redstoneblock on top of it, and the piston activated.

What I expected to happen was...:
I expected the piston to do nothing.

What actually happened was...:

The piston activated when it shouldn't have.
Steps to Reproduce:
1. Place a upward facing piston.
2. Place a slime-block on-top of the piston.
3. Place a redstone-block on-top of the slime-block. 
4. Place redstone in-front of the piston.
5. The redstone activates the piston for a second, then returns to normal. This happens even though the redstone has no signal to it and shouldn't activate the piston.