问为什么这个FTDI函数为fthandle返回零？

EN

   static FT_STATUS displayDevicesMethod2(void)
   {
    FT_STATUS   ftStatus;
    FT_HANDLE   ftHandle = NULL;

    // Get and display the list of devices connected
    // First call FT_CreateDeviceInfoList to get the number of connected devices.
    // Then either call FT_GetDeviceInfoList or FT_GetDeviceInfoDetail to display device 
   info.
    // Device info: Flags (usb speed), device type (600 e.g.), device ID (vendor, 
 product), 
    handle for subsequent data access.
    DWORD numDevs = 0;
    ftStatus = FT_CreateDeviceInfoList(&numDevs);   // Build a list and return number 
    connected.
    if (FT_FAILED(ftStatus))
    {
        printf("Failed to create a device list, status = %d\n", ftStatus);
    }
    printf("Successfully created a device list.\n\tNumber of connected devices: %d\n", 
    numDevs);
    // Method 2: using FT_GetDeviceInfoDetail
    if (!FT_FAILED(ftStatus) && numDevs > 0)
    {
        ftHandle = NULL;
        DWORD Flags = 0;
        DWORD Type = 0;
        DWORD ID = 0;
        char SerialNumber[16] = { 0 };
        char Description[32] = { 0 };
        for(DWORD i = 0; i <numDevs; i++)
        {
            ftStatus = FT_GetDeviceInfoDetail(i, &Flags, &Type, &ID, NULL, SerialNumber, 
    Description, &ftHandle);
            if (!FT_FAILED(ftStatus))
            {
                printf("Device[%d] (using FT_GetDeviceInfoDetail)\n", i);
                printf("\tFlags: 0x%x %s | Type: %d | ID: 0x%08X | ftHandle=0x%p\n",
                    Flags,
                    Flags & FT_FLAGS_SUPERSPEED? "[USB 3]":
                    Flags & FT_FLAGS_HISPEED? "[USB 2]":
                    Flags & FT_FLAGS_OPENED? "[OPENED]": "",
                    Type,
                    ID,
                    ftHandle);
                printf("\tSerialNumber=%s\n", SerialNumber);
                printf("\tDescription=%s\n", Description);
            }
         }
      }
    return ftStatus;
    }