Invalid argument error while using umount() function

Question

I am trying to unmount a usb-storage device through a c++ code as follows.

  while(umount("/dev/sdc1/")!=0)
  {
    std::cout << "ERROR: " << strerror(errno) << std::endl;
    std::this_thread::sleep_for (std::chrono::seconds(2));
  }

But, I get an error saying: Invalid argument. What am I doing wrong?

Thanks in advance!