Why is WinUsb_WritePipe returning error 997 With asynchronous winusb io?

If I use WinUsb_WritePipe with a LPOVERLAPPED entry, and use WinUsb_GetOverlappedResult to wait on the results, I have an error where WinUsb_WritePipe always returns false, and GetLastError returns error 997.

    OVERLAPPED ovr = { 0 };
    ovr.hEvent = CreateEvent(NULL, TRUE, FALSE, "USBWrite");
...
    while( ... )
    {
        // If waiting...
        WinUsb_GetOverlappedResult( winusbhandle, &ovr, &NumberOfBytesTransferred, 1);
        // Always,
        WinUsb_WritePipe(winusbhandle, 0x01, usb_out_buffer, sizeof(usb_out_buffer), &bytesWritten, &ovr);
        // This returns as a failure and GetLastError() == 997 
    }
}