diff --git a/include/wx/ffile.h b/include/wx/ffile.h
index 24d0437fba..d2957097ad 100644
--- a/include/wx/ffile.h
+++ b/include/wx/ffile.h
@@ -67,7 +67,7 @@ public:
     // returns true on success
   bool Write(const wxString& s, wxMBConv& conv = wxConvLibc)
   {
-      wxWX2MBbuf buf = s.mb_str();
+      wxCharBuffer buf = s.mb_str(conv);
       size_t size = strlen(buf);
       return Write((const char *) buf, size) == size;
   }
diff --git a/include/wx/string.h b/include/wx/string.h
index 6d8312f2ad..2858280b3a 100644
--- a/include/wx/string.h
+++ b/include/wx/string.h
@@ -452,7 +452,7 @@ public:
         { return m_pchData; }
     const wxWX2MBbuf mbc_str() const { return mb_str(*wxConvCurrent); }
 #else // !mmultibyte
-    const wxChar* mb_str() const { return m_pchData; }
+    const wxChar* mb_str(wxMBConv& WXUNUSED(con) = wxConvLibc) const { return m_pchData; }
     const wxWX2MBbuf mbc_str() const { return mb_str(); }
 #endif // multibyte/!multibyte
 #if wxUSE_WCHAR_T