running cuFFTXT library on two GPUs

I am working with CUDA 6.5:

Though when I run the sample simpleCUFFT_MGPU

result = cufftXtSetGPUs (plan_input, nGPUs, whichGPUs);

I get:

CUFFT_INVALID_DEVICE An invalid GPU index was specified.

Though when I run

int nGPUs;

I get 2 CUDA Devices.

I run this code over a SLI connection.

Anybody know what is going on, and how I can resolve this?