Error while creating site collection "user doesn't exist"

HI

I am creating new site collection diagrammatically  with host header.Following is the code

                      webapp.Sites.Add("http://user.test.local/sites/team", "test", "test", 1033,  "STS#0", "domain\\administrator", string.Empty, string.Empty, string.Empty, string.Empty, string.Empty, true);

I have run the above code under elevated privileges also but I am getting error "user doesn't exist or not found"

What configuration do I need to do?help me out