Differences in plugin functionality on PostOpportunityWin between CRM 2011 and CRM 2013

Hello,

I have developed a plugin for CRM 2011 (using the CRM Developer Toolkit for Visual Studio 2012) that executes when an opportunity is closed as "Won." The plugin itself is very simple, I retrieve the OpportunityClose object from the InputParameters, and then pull the opportunityid attribute from that entity and pass it off to a web service. For Dynamics CRM 2011, this works fine- for Dynamics CRM 2013, the Id property on the OpportunityClose entity is an empty GUID.

Sample code:

        protected void ExecutePostOpportunityWin(LocalPluginContext localContext)
        {
            if (localContext == null)
            {
                throw new ArgumentNullException("localContext");
            }

            var oc = (Entity)localContext.PluginExecutionContext.InputParameters["OpportunityClose"];
            var opportunityId = ((EntityReference)oc.Attributes["opportunityid"]).Id;
            var additionalProperties = new Dictionary<string, object>()
            {
                { "OpportunityCloseId", oc.Id }
            };
            // (snip) code to pass values to web service
        }

Following the above code, as I step through in a debugger... when integrating with Dynamics CRM 2013, oc.Id is an empty GUID. In Dynamics CRM 2011, it is the correct value. I've done a number of searches and read a lot of documentation, but nothing I've found indicates that this is either a bug or an intentional change.

Thanks in advance,

Steve