Casting - Cast MembershipUser To Custom Class (ASP.NET)?

I'm using the default SqlMembershipProvider,but I've created a custom MembershipUser class (SoeMembershipUser) because I needed a "DisplayName" property.All the DisplayName does is look at the UserName and format it differently.When I try to cast a MembershipUser to a SoeMembershipUser user I get an InvalidCastException.

Exact error is:
"Unable to cast object of type 'System.Web.Security.MembershipUser' to type 'Soe.Tracker.SoeMembershipUser'."

Here is the code that fails:

SoeMembershipUser user = (SoeMembershipUser)Membership.GetUser(username); // invalid cast

I have also tried casting later like so:

MembershipUser user = Membershipship.GetUser(username); // ok
...
string displayName = ((SoeMembershipUser)user).DisplayName; // invalid cast

Here is the SoeMembershipUser class:

NOTE: I left off the constructors at first, but added them in later when I started having problems. Adding them made no difference.

public class SoeMembershipUser : MembershipUser
{
public SoeMembershipUser()
: base()

[code]...