Queue full for Signalr with Redis backplane

Sometimes we get the following error on with the Signalr implementation using Redis as backplane:

System.InvalidOperationException: The queue is full.

at Microsoft.AspNet.SignalR.Messaging.ScaleoutStream.Send(Func`2 send, Object state)

at Microsoft.AspNet.SignalR.Infrastructure.Connection.Send(ConnectionMessage message)

at Microsoft.AspNet.SignalR.ConnectionExtensions.Send(IConnection connection, String connectionId, Object value)

Anyone know whats going on here? Is this the MaxQueueLength that is limiting for scaleout or something else?