为什么对话框DoModal返回-1解决办法

为什么对话框DoModal返回-1
Mydialog类里有enum { IDD = IDD_PROPPAGE_SMALL };

C/C++ code

void CMyView::OnLButtonDblClk(UINT nFlags, CPoint point){    // TODO: Add your message handler code here and/or call default    int i=dlg1.DoModal();    if(i==-1)MessageBox(_T("error"),NULL,0);        CView::OnLButtonDblClk(nFlags, point);}