diff --git a/src/msw/pen.cpp b/src/msw/pen.cpp
index cc3ae705aa..a5199247b2 100644
--- a/src/msw/pen.cpp
+++ b/src/msw/pen.cpp
@@ -31,6 +31,8 @@
 
 #include "wx/msw/private.h"
 
+int wx2msPenStyle(int wx_style);
+
 IMPLEMENT_DYNAMIC_CLASS(wxPen, wxGDIObject)
 
 wxPenRefData::wxPenRefData()