SubclassDlgItem on Property Page: failing on ASSERT

I'm trying to dynamically subclass a field on a property page.  The code looks like this:

  BOOL CMyPropertyPage::OnInitDialog()
  {
    CPropertyPage::OnInitDialog();
    m_myedithandler.SubclassDlgItem(IDC_MYEDIT,this);
    return TRUE;  
  }

The SubclassDlgItem() fails on an ASSERT in

  BOOL CWnd::Attach(HWND hWndNew)
  {
    ASSERT(m_hWnd == NULL);                       //OK
    ASSERT(FromHandlePermanent(hWndNew) == NULL); //fails
    ...

FromHandlePermanent returns a non-NULL pWnd, presumbably because the control in question is already in the permanent map.

I've used the same override logic on an edit field in a regular dialog (derived from CDialog rather than CPropertyPage) without any problem.  Is there something special about subclassing on a property page?  Or can someone point me to something else I am doing wrong?