I'm assuming you're not on the last phase. It's not bugged.
The first time he throws it, you have to use a special attack into it. Then you get stacks. Once you have stacks, you can either parry or heavy attack into it.
You need to have the green removal charge first. Special into his blast to get it then you can parry the next blast.
What Rafikii said, you have to throw a special to get a removal charge, looks like a green icon in your HUD