How to add SPUser to sharepoint list field (lookup)

Question

I have a sharepoint list, with a lookupfield for users. The column name is "SystemUser". I have successfully added a listitem by using javascript, setting the "SystemUser" to the current user by calling web.get_currentUser();

I want to set another user to the field "Users". I get the users list (web.get_siteUserInfoList()), and retrieve the user with a given ID. When the user is loaded, I set the loaded user as a value to the field "SystemUser".

But this approach does not work. I get the following errormessage (translated):

"An error occured: Invalid data is used to update the listelement. The field you tried to update could be writeprotected. Undefined";

Any ideas? Here's the code:

function () {
window.dialog = $(this);
var context = SP.ClientContext.get_current();
var web = context.get_web();
var list = web.get_lists().getByTitle('MyList');
var listItemCreationInformation = new SP.ListItemCreationInformation();
var listItem = list.addItem(listItemCreationInformation);

var name = $('#<%=txtbxName.ClientID %>').val();
var selectedUserID = $('#<%=hiddenUserFieldID.ClientID %>').val();

var userList = web.get_siteUserInfoList();
var selectedUser = userList.getItemById(selectedUserID);
context.load(selectedUser);

context.executeQueryAsync(function () {
    listItem.set_item('SystemUser', selectedUser);
    listItem.update();

    context.executeQueryAsync(function () {
        window.dialog.dialog('close');
        SP.UI.Notify.addNotification('Added...');
        }, 
        function (sender, args) {
            alert('Inner exception: ' + args.get_message() + '\n' + args.get_stackTrace());
       });

        }, function (sender, args) {
            alert('Outer exception: ' + args.get_message() + '\n' + args.get_stackTrace());
        });
        }

Answer 1

Instead of writing the selectedUser (which I think is an SPUser object?) you should write a SPUserLookupValue, or it's string equivalent which is ID;#Name, i.e. 1;#David Lozzi.