Membership.ValidateUser() doesn't work.

Hello,
I'm getting started with the membershipProvider. I have some problems with the ValidateUser() method of the Membership static class.

I first created a MembershipUser  :
MembershipCreateStatus status = MembershipCreateStatus.UserRejected;
MembershipUser user = Membership.CreateUser("name", "apassword", "email@domaine.com",
"aquestion", "an answer", false, out status);
if (status == MembershipCreateStatus.Success)
MessageBox.Show("Création OK");  
It works : I get an alert "Création OK".

And then I tested the ValidateUser() method :


Membership.ValidateUser("name", ...