diff --git a/include/wx/filename.h b/include/wx/filename.h
index f78164d73e..5d316c52b8 100644
--- a/include/wx/filename.h
+++ b/include/wx/filename.h
@@ -85,7 +85,7 @@ enum
 };
 
 // error code of wxFileName::GetSize()
-extern wxULongLong wxInvalidSize;
+extern WXDLLIMPEXP_DATA_BASE(wxULongLong) wxInvalidSize;