NetworkTransport.Send: Cannot send message of 0 bytes length

With 5.1.1f1 PC, how would we get into a state with the HLAPI where the LLAPI thinks we are trying to send 0 bytes via NetworkServer.SendWriterToReady(null, writer, 2);?
We check that the writer always has more than 0 bytes before we send the message.

This repeats for a long time on our server:
2015-09-09 17:39:33.699+00:00 [ERR] Cannot send message of 0 bytes length
2015-09-09 17:39:33.699+00:00 [ERR] Failed to send internal buffer conn:1 channel:2 bytesToSend:0
2015-09-09...

NetworkTransport.Send: Cannot send message of 0 bytes length