diff --git a/include/wx/sizer.h b/include/wx/sizer.h
index 8ae125062d..125dd55c7f 100644
--- a/include/wx/sizer.h
+++ b/include/wx/sizer.h
@@ -799,6 +799,7 @@ public:
     wxBoxSizer(int orient)
     {
         m_orient = orient;
+        m_totalProportion = 0;
 
         wxASSERT_MSG( m_orient == wxHORIZONTAL || m_orient == wxVERTICAL,
                       _T("invalid value for wxBoxSizer orientation") );