include user data in query - asp.net

Question

I would like to query a table, which holds workflow events with the appr. user id. And instead of the user id, I would like to show the user name.
I have this model:
public class WorkflowEvent
{
public long Id { get; set; }
public DateTime EventDate { get; set; }
public WorkflowStatus Status { get; set; }
public int UserId { get; set; }
public AppUser User { get; }
public string Note { get; set; }
public long WorkflowId { get; set; }
public long DocumentId { get; set; }
}
And this is the user model (it is the standard IdentityUser, I use Asp.Net Identity):
public class AppUser : IdentityUser<int>
{
public string RealName { get; set; }
public string AppTheme { get; set; }
}
And this is my query:
public IEnumerable<WorkflowEvent> WorkflowEvents(int DocumentId)
{
return DataContext.WorkflowEvents.Where(e => e.DocumentId == DocumentId).Include(e => e.User).OrderBy(e => e.EventDate);
}
Unfortunatelly, I get following error on this: System.InvalidOperationException: Lambda expression used inside Include is not valid. In the WorkflowEvents table, there is a foreign key on the UserId, of course.
Have you any idea, why do I get this error message? What goes wrong?

Answer 1

Both Sami's and Nik's suggestions helped. I added the setter to the AppUser. And then I got the error, that the AppUser object isn't exists. This due to the fact, that I have in my app two datacontexts: one for the domain models and one for Identity. So, I just add this line of code to the domain data context:
public DbSet<AppUser> AspNetUsers { get; set; }
And everything worked like a charm.