MembershipCreateStatus returns Invalid UserName

Hi,

I'm using a custom membership provider (XML file), and I want to insert the user's credentials for more than thousand users by using CreateUser method
the problem comes when I try to insert a new user the MembershipCreateStatus returns Invalid UserName
here is my code:

string employeeID="1234567"

MembershipCreateStatus status;
MembershipUser newUser =Membership.Provider.CreateUser(
employeeID, employeeID,
"aa@aa.aa", String.Empty, String.Empty, true,
Guid.NewGuid(), out status);

if (newUser == null) errorMsg = status.ToString();
else Membership.UpdateUser(newUser);  
why the MembershipCreateStatu ...