why wxScreenDC get black screen

Question

Does anyone know why wxScreenDC only get black screen when Blit to a wxMemoryDC.
I'm using Debian system with wxWidgets v3.1.5(or v3.1.6)
Same code, replace wxScreenDC with wxWindowDC everything go fine.
This is my code:

void MyFrame::OnCapture(wxCommandEvent &event)
{
    wxFileDialog dlg(this, _("Save PNG file"), "", "",
                       "PNG files (*.png)|*.png", wxFD_SAVE|wxFD_OVERWRITE_PROMPT);

    if (dlg.ShowModal() == wxID_CANCEL)return;

    wxScreenDC dc;
//  wxWindowDC dc(this);
    wxSize sz = dc.GetSize();
    wxBitmap bmp(sz.x, sz.y);

    wxMemoryDC memDC;
    memDC.SelectObject(bmp);
    memDC.Blit(0, 0, sz.x, sz.y, &dc, 0, 0);
    memDC.SelectObject(wxNullBitmap);
    bmp.SaveFile(dlg.GetPath(), wxBITMAP_TYPE_PNG);
}