parallel_reduce problem

I know I must be doing something really dumb, but I have not been able to figure out what. I'm basically just doing the simple reduction example in the TBB book, but using the IPP sum routine (sum of all pixels in a Ipp16u plane). Sounds simple enough, but it looks like join() is not being called enough times. Do I ever need to explicitly call join() or does the system always call it?