Help with pudding, please

I've made this recipe http://smittenkitchen.com/2008/02/best-chocolate-pudding/ probably 20 times and it's never failed me. This time, however, I added maybe 1/2 cup of nutella along with the chocolate, and even after several hours in the fridge, it wouldn't set up. Does anyone know why the nutella would cause it to stay runny? I don't mean slightly softer, I mean like soup....