The attribute cannot be modified because it is owned by the system

Trying to utilize a virtual attribute of type DN and pump the values of a users "memberof" attribute into it at the time the virtual attribute is accessed.  However, continuing to receive the following error: The attribute cannot be modified because it is owned by the system.  Any thoughts?

Sub onPreGet(Request)

    Const ADS_PROPERTY_UPDATE = 2

    Const EDS_PROPERTY_APPEND = 3

    groupAttribute = "edsvaMemberOfReplica"

    If Request.Class <> "user" Then Exit Sub

    If Not Request.IsAttributeRequested(groupAttribute) Then Exit Sub   

    Set user = GetObject(Request.ADsPath)

    memberOf = user.GetEx("memberOf")

    user.GetInfoEx Array(groupAttribute,"isSingleValued"),0

      user.PutEx ADS_PROPERTY_UPDATE, groupAttribute, memberOf

      user.Put "isSingleValued",False

      user.SetInfo

      Set user = Nothing   

End Sub