A question on Gtkmm's

I would like to add a signal to a Gtk::Entry object, but for some
reason this gives me bunch of errors saying that it is

D:\XWin\include\sigc++-2.0\sigc++\adaptors\adaptor_trait.h|84|error:
no match for call to `(sigc::bound_mem_functor0<bool, jv+e::Entry>)
(GdkEventFocus* const&)'

I using Gtkmm 2.10.x for Windows
Thanks in advance!

Can any body help?!
namespace jve{
class Entry: virtual public Gtk::VBox {

      .......
      Gtk::Entry* text_box;
public:
      Entry();
      .........
      void onBlanck();
};
}
jve::Entry::Entry(){
      ....
      text_box  = Gtk::manage(new Gtk::Entry());
      text_box->signal_focus_out_event().connect(sigc::mem_fun(*this,
&Entry::onBlank)); <== here is the problem
      ....
}
bool jve::Entry::onBlank(){
      ....
      // Display a Dialogbox explaining the problem
      ....
      //! recapture the focus
      text_box->grab_focus();
      return false
}