Banana Nut Cake

1 pkg yellow cake mix
1 pkg (3.4 oz) instant banana cream pudding
1/4 cup vegetable oil
4 eggs
1 cup water
1 1/2 cups mashed bananas (about 2 medium)
3/4 cup chopped walnuts

In bowl combine first 5 ingredients. Beat on medium speed 2 minutes. Add bananas; mix well. Stir in nuts.
Pour in greased 9x13 pan and bake at 350 degrees for 50 to 55 minutes. Cool on wire rack. Dust with powdered sugar if desired.