WM_NOTIFY and missing NMHDR

I have a property sheet with a ListView and a few textboxes.  I am attempting to catch the event when the user clicks on the ListView and determine what item they clicked.  To do this, I am using the WM_NOTIFY event.  The WM_NOTIFY fires when the ListView is clicked, but the lParam parameter points to an invalid memory location.

INT_PTR CALLBACK DialogProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
      switch(uMsg)
      {
      case WM_NOTIFY:
            {
                  //LPNMHDR pnmh = (LPNMHDR)lParam;
                  //switch(pnmh->code)
                  LPNMITEMACTIVATE lpn = (LPNMITEMACTIVATE)lParam;
                  switch(lpn->hdr.code)
                  {
                  case LVN_ITEMACTIVATE:
                        {
                              switch(lpn->hdr.idFrom)
                              {
                              case IDC_USED_ADDR:
                                    {
                                          OnUsedAddr(hWnd, lParam);
                                          return FALSE;
                                    }
                              }
                        }
                  }
            }
      }
      return FALSE;
}

Casting the lParam as an LPNMITEMACTIVATE makes no difference, the memory location is invalid.  When attempting to run in debug mode, the application breaks at switch(lpn->hdr.code) with the message "Unhandled exception at 0x0041425e in cfgtest.exe:  0xC0000005:  Access violation reading location 0x001e0e84."  0X001E0E84 is the value contained in lParam and when examined in the memory watch window shows up as ?? ?? ?? ??...

Is there something that I'm doing wrong that my NMHDR is not showing up, or is there some way that I need to check to ensure that it's valid before using it?