VIDIOC_DQBUF gives i/o error

[Eagle Jones <eagle@xxxxxxxxxxxx>]

I'm having a problem getting v4l2 to capture, using the bttv 0.8.38
driver. I have enumerated the device, and manage to queue my buffers up
ok, and start the streaming without error. But then I do the following:

...
      FD_ZERO(&fds);
      FD_SET(vid, &fds);
      tv.tv_sec = 10;
      tv.tv_usec =0;
      i = select(vid + 1, &fds, 0, 0, &tv);
      if(i == -1)
	perror("select error");
      if(i == 0) 
	perror("select timeout");

      if(FD_ISSET(vid, &fds))
	{
	  vb.type = V4L2_BUF_TYPE_CAPTURE;
	  if(ioctl(vid, VIDIOC_DQBUF, &vb) < 0)
	    {
	      perror("couldn't dq buf");
	      return -1;
	    }
...

When I run, it gives me:
couldn't dq buf: Input/output error

Why does DQBUF return -IOERR?

Eagle