I made the whole recipe (there were 5 of us) and added chopped walnuts.
From others, I learned the the dough could be a bit sticky. I made sure I had all of the mixtures ready (wet and dry) and the butter was really cold before mixing it all together. My dough was a bit sticky, but when I rolled it out on the floured board, it was perfect.
We loved the topping and agreed that they needed that little bit of sweet.