Why would BufferedStream.Write throw "This stream does not support seek operations"?

This one puzzles me. I get an error about seek when I'm not even calling it?

I have code that looks something like this:

// send 42
uint value = 42;
byte[] msg = BitConverter.GetBytes(value);
stream.Write(msg, 0, sizeof(uint));

and I get this exception:

System.NotSupportedException was unhandled
Message="This stream does not support seek operations."
Source="System"
StackTrace:
   at System.Net.Sockets.NetworkStream.Seek(Int64 offset, SeekOrigin origin)
   at System.IO.BufferedStream.FlushRead()
   at System.IO.BufferedStream.Write(Byte[] array, Int32 offset, Int32 count)
...

stream is of type System.IO.BufferedStream. What could possibly be going on?

edit with more info:

sizeof(uint)==msg.length in this case.
The stream is declared as stream = new BufferedStream(new NetworkStream(socket), 1024)

edit:

That was it! While one can read and write on a single NetworkStream, when switching to a BufferedStream it is necessary to have a separate one for reading and writing. One can apparently just call the NetworkStream constructor twice on the same socket to get that.

I'd accept both Justin and Hans' answers if I could, because one let me exactly understand what was wrong, and the other led me to the solution. Thanks everyone!