Redstone signal being passed through glass

When building a redstone clock, I noticed that the sticky piston in the back was receiving a redstone signal when it should not have been. The signal was being transmitted through a glass block, which should stop the signal. Once that block is broken and replaced, though, the piston returns to it's normal, non-extended form. I have attached pictures of what happens and what should happen when it is in front of a glass block.