Auto Share opportunity record to specific users

Hi CRM Support,

I'm working on a code that will auto share opportunity record to specific number of users. I have it work correctly when i set a specific id of a user. However i'm experiencing an error when i fetch specific users from another entity.. error shows the following message 

Please see my plug-in codes below:

                        //grant access test codes-------------------------------------------------------------------------------START

                        //Fetch Users to that will have access to the opportunity
                        QueryExpression queryLeadcodeAssignee = new QueryExpression("nexus_nexus_leadcode_systemuser");
                        queryLeadcodeAssignee.ColumnSet = new ColumnSet(new string[] { "systemuserid" });
                        queryLeadcodeAssignee.Criteria.AddCondition("nexus_leadcodeid", ConditionOperator.Equal, ((EntityReference)PostImageForOpportunities["nexus_leadcode_lookup"]).Id);

                        queryLeadcodeAssignee.PageInfo.ReturnTotalRecordCount = true;
                        EntityCollection LeadcodeAssigneeEntity = service.RetrieveMultiple(queryLeadcodeAssignee);

                        foreach (Entity LeadcodeAssigneeEntityResult in LeadcodeAssigneeEntity.Entities)
                        {
                        
                            var Rights = AccessRights.ReadAccess | AccessRights.WriteAccess ;

                            var sampleuser1 = new EntityReference("systemuser", ((EntityReference)LeadcodeAssigneeEntityResult["systemuserid"]).Id);//new Guid("E9007311-358D-E211-A12C-36D7C49CDBE8"));

                            var principalAccess = new PrincipalAccess
                            {
                                // Gives the principal read write access
                                AccessMask = Rights,

                                // Set the PrincipalAccess Object's Properties
                                Principal = sampleuser1
                            };

                            // Create the Request Object
                            var grantAcessRequest = new GrantAccessRequest();
                            // Set the Request Object's properties
                            grantAcessRequest.PrincipalAccess = principalAccess;
                            // Set the Target. In my case it is account record
                            var entityReference = new EntityReference("opportunity", PostImageForOpportunities.Id);
                            //throw new InvalidPluginExecutionException("EntityReference");
                            grantAcessRequest.Target = entityReference;

                            //Execute the Request
                            service.Execute(grantAcessRequest);
                        }
                        //grant access test codes-------------------------------------------------------------------------------END

Advance thank you for you help..

Regards,

Honey Dulatre