bug with kassadin's force pulse

I'm not sure if this is a bug or not but the slow from his force pulse cone does not match the complete animation. I was playing against a kassadin and every time I was just outside of his animation for force pulse I would still get slowed but didn't take any damage which allowed the opposing team to catch up with no problem. I don't know if this is just a visual bug or what not since I do not use kassadin that often to know its range.