Object error on my custom HTML

Hi Everybody,

I have very basic question on my simple code.

i am just trying to pull up User name , but i am getting 'Object Error'.
I am getting same error on whatever i am trying to use.
i think somewhere wrong i am doing.
Can you please advice me?.

       I have placed this sample code in my custom HTML.

      CODE:

                    try

                        {

                             var userName = nlapiGetUser();

                            alert(userName);

                        }

                    catch(err)

                     {

                         alert(err);

                     }



   I am getting the error like:[object Error].