RegisterRawInputDevices() is generating ERROR_FILE_NOT_FOUND

When my windows application (a game) starts, I register it to receive raw input messages:

RAWINPUTDEVICE Rid[2];
Rid[0].usUsagePage  = HID_USAGE_PAGE_GENERIC;     // 0x01
Rid[0].usUsage      = HID_USAGE_GENERIC_MOUSE;    // 0x02
Rid[0].dwFlags      = RIDEV_INPUTSINK;   
Rid[0].hwndTarget   = sMainWindow;                // HWND from created window

Rid[1].usUsagePage  = HID_USAGE_PAGE_GENERIC;     // 0x01
Rid[1].usUsage      = HID_USAGE_GENERIC_KEYBOARD; // 0x06
Rid[1].dwFlags      = RIDEV_INPUTSINK;   
Rid[1].hwndTarget   = sMainWindow;

BOOL result = RegisterRawInputDevices( Rid, 2, sizeof( Rid[0] ) );

if( !result )
{
    EGSystemError( "RegisterRawInputDevices Error: ", GetLastError() );
}

This game has been released and many users seem to have no trouble with it, but I have one user who is getting the error message:

"RegisterRawInputDevices Error: The system cannot find the file specified"

So, the question is- why would RegisterRawInputDevices() ever generate a ERROR_FILE_NOT_FOUND error? I haven't seen any references in the docs that help. Does anyone have experience with this?