Debatable under the current rules - the Leicester penalty at Newcastle was very similar.

I actually think that if your arm is in the air and the ball hits it, preventing it from heading towards goal in this case, it should be a penalty. Bit like when the ball hits your foot in hockey. Intention doesn't come into it, the defender in hockey just has a responsibility not to interfere with the ball with his/her foot and is penalised for mistakes. I think that's how it works anyway - can appear harsh at times but it's clear and consistent.