What is causing a SignalR FormatException Invalid Cursor? - asp.net-mvc

Question

I have written a simple SignalR hub:
public void PostUpdate(string message)
{
try
{
var update = URS.Tools.JsonSerializer.DeserializeFromJson<DataUpdate>(message);
switch (update.UpdateType)
{
case DataUpdateType.Opportunity:
Clients.All.UpdateOpportunity(update.DataUpdateId);
break;
case DataUpdateType.Contact:
Clients.All.UpdateContact(update.DataUpdateId);
break;
case DataUpdateType.Client:
Clients.All.UpdateClient(update.DataUpdateId);
break;
}
}
catch (Exception exception)
{
Logger.LogError(exception);
}
}
It is running with a Silverlight 5 client. For the most part it works, but after 5 - 30 minutes I get an error coming from withing the library:
Exception information:
Exception type: FormatException
Exception message: Invalid cursor.
at Microsoft.AspNet.SignalR.Messaging.Cursor.GetCursors(String cursor, Func`2 keyMaximizer)
at Microsoft.AspNet.SignalR.Messaging.DefaultSubscription..ctor(String identity, IEnumerable`1 eventKeys, TopicLookup topics, String cursor, Func`2 callback, Int32 maxMessages, IStringMinifier stringMinifier, IPerformanceCounterManager counters)
at Microsoft.AspNet.SignalR.Messaging.MessageBus.CreateSubscription(ISubscriber subscriber, String cursor, Func`2 callback, Int32 messageBufferSize)
at Microsoft.AspNet.SignalR.Messaging.MessageBus.Subscribe(ISubscriber subscriber, String cursor, Func`2 callback, Int32 maxMessages)
at Microsoft.AspNet.SignalR.Infrastructure.Connection.Receive(String messageId, Func`2 callback, Int32 maxMessages)
at Microsoft.AspNet.SignalR.Transports.ForeverTransport.ProcessMessages(ITransportConnection connection, Func`1 postReceive, Action`1 endRequest)
When I look at the packets, I cannot see a difference between the one just prior to the error that succeeds and the call that fails. It is running serverEvents transport, but I tried longPolling to no avail.
The exception is happening in the server.
Any ideas what could be causing the above error and/or how to resolve it?