DirectoryEntry and msRADIUSFramedIPAddress

[Davide Rimondi]

Hello,
I would like create a simple user and setting a static IP to him. but every time I try to set msRADIUSFramedI PAddress property i have this message of error:
System.Runtime. InteropServices .COMException (0x8000500F): The directory property cannot be found in the cache.
Please help me (and sorry my english).

 public void createNewUser(String userName,String Password,String staticIp)
        {
            try
            {
                string s_Username = userName;
                string s_Password = Password;
                string s_Description = "Test User Account";

                DirectoryEntry AD = new DirectoryEntry("WinNT://" + Environment.MachineName + ",computer");
                // create a new user
                DirectoryEntry NewUser = AD.Children.Add(s_Username, "user");
                // set the properties
                NewUser.Invoke("SetPassword", new object[] { s_Password });
                NewUser.Invoke("Put", new object[] { "Description", s_Description });
                 NewUser.Properties["msRADIUSFramedIPAddress"].Add(staticIp);           

                // commit the changes
                NewUser.CommitChanges();
                txtResult.Text = "Success!!";
            }
            catch (Exception err)
            {
                // set the error message
                txtResult.Text = err.ToString();
            }
        }

Davide