Optix Prime error with rtpQueryExecute

I am trying to run a query on a CPU context and I get the following error:

Optix: Function “_rtpQueryExecute” caught exception: Cannot create new CPU traverser thread., [65667198]

Windows 8.1, x32, CUDA 6, Optix 3.6.2.

This error seems to have gone after I converted my application from x32 to x64. Can anyone explain why?