Thanks Shashank.

To clarify my example, I'm talking about the average precision at 10 if results 1 through 5 are relevant but results 6 through 10 are not relevant.

I was using this definition:

https://en.wikipedia.org/wiki/Evaluation_measures_(information_retrieval)#Average_precision

It's possible that I messed up my calculation, but I'm quite sure the average precision at 10 is less than 1 in this case.