Setting an entity reference in a plugin

I'm trying to set an entity reference from within a plugin. It's on the associate of two entities - stock and application form. When a stock item is associated with an application form, I get the name of the applicant (which is another entity - Contact) from a look up on the application form, and populate a look up on the stock item with that contact.

Here is my code:

        private const string ASSOCIATE = "Associate";
        private const string DISASSOCIATE = "Disassociate";

        public void Execute(IServiceProvider serviceProvider)
        {
            try
            {
              
                PluginSetup(serviceProvider);

                if (context.MessageName == ASSOCIATE || context.MessageName == DISASSOCIATE)
                {
                    // Get the Relationship Key from context
                    if (context.InputParameters.Contains("Relationship"))
                    {
                        string relationshipName = context.InputParameters["Relationship"].ToString();
                        // Check the Relationship Name with intended one
                       if (relationshipName == "applicationForm_stock")
                        {
                            if (context.MessageName == ASSOCIATE)
                            {
                                SaveAssociatedStock(ASSOCIATE);
                            }
                            else if (context.MessageName == DISASSOCIATE)
                            {
                                SaveAssociatedStock(DISASSOCIATE);
                            }
                        }
                        else
                        {
                            return;
                        }
                    }
                }

            }
            catch (Exception ex)
            {
                throw new InvalidPluginExecutionException("An error occurred");
            }
        }

        private void SaveAssociatedStock(string messageType)
        {
            try
            {
                EntityReference applicationFormEntity = null;
                EntityReferenceCollection stockEntities = null;
                EntityReference contactEntityReference = null;
                Entity stockItem = new Entity();

                ColumnSet nameCol = new ColumnSet();
                nameCol.AddColumn("name");
                ColumnSet contactNameCol = new ColumnSet();
                contactNameCol.AddColumn("contactname");

                // Get Application Form entity reference from Target Key from context
                if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is EntityReference)
                {
                    applicationFormEntity = (EntityReference)context.InputParameters["Target"];
                    Trace("applicationFormEntity.Id: " + applicationFormEntity.Id);
                    Entity test = service.Retrieve("applicationForm", applicationFormEntity.Id, contactNameCol);
					// Get the contact name from the contact lookup on the application form
                    contactEntityReference = (EntityReference)test.Attributes["contactname"];                   
                }
                // Get Stock reference from RelatedEntities Key from context
                if (context.InputParameters.Contains("RelatedEntities") && context.InputParameters["RelatedEntities"] is EntityReferenceCollection)
                {
                    stockEntities = context.InputParameters["RelatedEntities"] as EntityReferenceCollection;
                    for (int i = 0; i < stockEntities.Count; i++)
                    {
                        // Get name of current Stock item  
                        stockItem = service.Retrieve("stock", stockEntities[i].Id, nameCol);

                        if (messageType == ASSOCIATE)
                        {
                            // Add the contact to the current Stock item record
                            Entity updateStockItem = new Entity("stock");
                            updateStockItem.Attributes.Add(new KeyValuePair<string, object>("stockid", stockEntities[i].Id));

                            updateStockItem.Attributes.Add(new KeyValuePair<string, object>("owner", contactEntityReference.Id));
                            service.Update(updateStockItem);
                        }
                        else if (messageType == DISASSOCIATE)
                        {
                            // Remove the contact from the current Stock item record
                            Entity updateStockItem = new Entity("stock");
                            updateStockItem.Attributes.Add(new KeyValuePair<string, object>("stockid", stockEntities[i].Id));

                            updateStockItem.Attributes.Add(new KeyValuePair<string, object>("owner", null));
                            service.Update(updateStockItem);
                        }
                    }
                }

However, I'm getting this error:

 <Message>An error occurred in the AssociateEntities.AssociateEntities.Execute plug-in.An error occurred in the AssociateEntities.AssociateEntities.SaveAssociatedStock plug-in.System.InvalidCastException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #863F0E66</Message>

All my IDs have valid values in them, as I've already put in trace logging, and can see that they are correct.

Thanks.