Am having error with authentication code

Am having error on this line of code of my authentication,

if (user. Identity.Isauthenticated)
{
var username = membership. GetUser (User. Identity.Name).ProviderUserKey;

}
Bit if I comment on the code below it will be working very fine.:

var username = membership. GetUser (User. Identity.Name).ProviderUserKey;

What what's the solution?