"Collection was of a fixed size" Exception in EF4 with POCO

Question

I am using EF4 with WCF and POCO. I removed all virtual keyword in POCO entities.

I have Employee and Team entities and relationship between both is 1:N, means that one employee can be allocated only one team.

And I'd like to add new employee with existing team. Following code is in the client side.

private void btnAdd_Click(object sender, RoutedEventArgs e)
        {
            Team teamFromDb = ServiceProxy.GetService.GetTeamById(181);
            Employee newEmp = new Employee{ UserName="username"};
            newEmp.Team = teamFromDb;
            ServiceProxy.GetService.AddEmployee(newEmp);                
        }

Following code is in the server side (Dao layer)

public void AddEmployee(Employee emp)
        {
            ctx.Employees.AddObject(emp);
        }

        public Team GetTeamById(int teamId)
        {
            return ctx.Teams.Where(t => t.TeamId == teamId).FirstOrDefault();
        }

Problem is that I got "Collection was of a fixed size" Exception when I add teamFromDb instance to the newEmp.Team property in the client code.

Do I need to add some more code to fix?

In addition, What do I need to for Insert/Update/Delete job with POCO classes and WCF

Thanks in advance.