piston and redston block bug

Put the summary of the bug you're having here

when i destroyed a redstone block which powers a stiky piston it will turn off

What actually happened was...:
but they will not turn off

Steps to Reproduce:
1. place a facing up stiky piston
2. place a redstone cube up it
3. power the stiky piston
4. the stiky piston will not turn off