when retrieving a Person field PropertyOrFieldNotInitializedException

I see an exception with quick watch:

 'listitems.ListItemCollectionPosition' threw an exception of 
type 'Microsoft.SharePoint.Client.PropertyOrFieldNotInitializedException'

Here is my code.

 Microsoft.SharePoint.Client.ListItemCollection listitems = list.GetItems(query);

             context.Load(listitems, items => items.Include(item => item , item => item["MyPerson"]));
             context.ExecuteQuery();

             User u = null;
             foreach (ListItem item in listitems) // a quick watch on listitems shows the exception
             {


                 FieldUserValue fuv = (FieldUserValue)item["MyPerson"];
                 u = context.Web.EnsureUser(fuv.LookupValue);
                 context.Load(u, user => user.Email);
                 context.ExecuteQuery();

                 res = u.Email;


             }

MyPerson corresponds to the internamname of a field, which is person or group