Problem with Same User Name in Different Application ID

I get an error only when Same User Name was created in Different Application ID.

Input string was not in a correct format.line- gluserID = Convert.ToString(command.ExecuteScalar())


Here is my code
If User.Identity.IsAuthenticated Then
            Dim UserIdentity As MembershipUser = Membership.Providers("CustMembershipProvider").GetUser(HttpContext.Current.User.Identity.Name, False)
            If UserIdentity IsNot Nothing Then
                'auth user
                Dim currentUserID As String = UserIdentity.ProviderUserKey.ToString()
            &nbs ...