Add user to a new list item

I would like to add to a list a new item with a User or Group field. The code is this:

 ListItemCreationInformation  listItemCreationInformation = new ListItemCreationInformation();

 ListItem item = list.AddItem(listItemCreationInformation);


    User u = context.Web.EnsureUser("Name Surname");

    item["MyUserField"] = u;
    item.Update();

Do you know what's wrong or what I should do?