diff --git a/include/wx/simplebook.h b/include/wx/simplebook.h
index d54ebc45b6..5ca253ac10 100644
--- a/include/wx/simplebook.h
+++ b/include/wx/simplebook.h
@@ -139,7 +139,7 @@ public:
 protected:
     virtual void UpdateSelectedPage(size_t newsel)
     {
-        m_selection = newsel;
+        m_selection = (int)newsel;
     }
 
     virtual wxBookCtrlEvent* CreatePageChangingEvent() const