Membership.GetUser always returns null

Hi i have a console application which uses a class library to return the user who has created an article.

the code in the class library is

MembershipUser thisUser ;
thisUser = Membership.GetUser ( creatorid ); // creator id is set beforehand and returns a valid users Guid 
if (thisUser != null)
{
school.Email = thisUser.Email;
school.CreationDate = thisUser.CreationDate;
}

I have in this class library created an app.config file to specify the membership provider. The code is ...