listbox.addstring error, why?

all i'm trying to do was use a dialog box to ask what to open an unknown file type as, and i'm fairly sure it is the add string that is doing it. when i comment them out (making it pointless) it works fine, minus the text.

if ((type != typeA) | (type != typeB) | (type != typeC))
{
COpenAs dlg;  // a dialog box with a list box and an ok and cancel button
               // and used the class wiz to link CListBox m_File_Type to
               // the dialog box list box no other noteable changes made
dlg.m_File_Type.AddString("Type A");  // *******puts error here
dlg.m_File_Type.AddString("Type B");
dlg.m_File_Type.AddString("Type C");
dlg.m_File_Type.SetCurSel(0);
if (dlg.DoModal() == IDOK)
  if (dlg.m_File_Type.GetCurSel() != LB_ERR)
    type = dlg.m_File_Type.GetCurSel() + 1;
}