Unusual piston and redstone block glitch

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
The piston would stay retracted
What actually happened was...:
Describe what happened here
the piston extended
Steps to Reproduce:
1. Redstone block on top
2. Anything(or air) in the middle
3. Piston adjacent to middle block