For any number "a" (except zero), a^{0}= 1.

Now I have a homework problem:

Simplify the expression: mn^{0}

So, I put in for my answer 1, but when it was graded the teacher said that that was the wrong answer. How could it be wrong if any number the the power if zero is 1? Did I misread my book, or did the teacher's answer key make a mistake?