Unity TCP async functions

I'm trying to get unity to connect to a TCP server using asynchronous functions. The gist of the call is:

 NetworkStream stream = tcpClient.GetStream ();
 byte[] bytes = Encoding.ASCII.GetBytes ("1");
 stream.BeginWrite (bytes, 0, bytes.Length, new AsyncCallback(sendCallback), stream);

where tcpClient is, as one would expect, a TcpClient. The sendCallback looks like this:

 public void sendCallback(IAsyncResult ar)
 {
     NetworkStream stream = (NetworkStream)ar.AsyncState;
     stream.EndWrite (ar);
 }

The server does not receive the data until I close the connection. In other words, it doesn't receive the data unless I add (inside the callback) the line:

 tcpClient.Close ();

Obviously this is undesirable. If I use synchronous send calls everything works fine. am I doing something wrong or is this a unity bug? If this is a unity bug, I guess my other option would be to go with synchronous calls on a separate thread. Any gotchas to expect there?