NvAPI_DRS_SaveSettings error

Silly question I guess but it is stopping me.

NvAPI_DRS_SaveSettings returns NVAPI_ERROR (-1)

Other functions work fine :

NvAPI_Initialize
NvAPI_DRS_CreateSession
NvAPI_DRS_LoadSettings
NvAPI_DRS_GetBaseProfile
NvAPI_DRS_SetSetting
NvAPI_DRS_DestroySession

I am trying to enable NVIDIA on an Optimus machine from my code :

http://onemanmmo.com/index.php?cmd=newsitem&comment=news.1.211.0

Same code as many others, and none have reported such an error.

Any ideas are most welcome.

Tested on Acer Aspire V3-571G, Windows 7 64bit.