cuInit() failed error 999 on K5100M 64-bit (32 bit works)

GPU: Quadro K5100M (Kepler) with driver 426.00. OS: Windows 8.1 Pro x64.

I built CUDA sample deviceQueryDrv against CUDA toolkit 8.0. I built 32-bit and 64-bit versions.

The 32-bit version runs fine. The 64-bit version fails cuInit(0) with error 999 (unknown error).

Your ideas for resolving this would be appreciated. Thank you.