diff --git a/include/wx/sstream.h b/include/wx/sstream.h
index f24419f8ad..47d17565a4 100644
--- a/include/wx/sstream.h
+++ b/include/wx/sstream.h
@@ -62,8 +62,8 @@ public:
     //
     // Note that the conversion object should have the life time greater than
     // this stream.
-    wxStringOutputStream(wxString *pString = NULL,
-                         wxMBConv& conv = wxConvUTF8);
+    explicit wxStringOutputStream(wxString *pString = NULL,
+                                  wxMBConv& conv = wxConvUTF8);
 
     // get the string containing current output
     const wxString& GetString() const { return *m_str; }
diff --git a/interface/wx/sstream.h b/interface/wx/sstream.h
index a1250e5ff8..e9522db36e 100644
--- a/interface/wx/sstream.h
+++ b/interface/wx/sstream.h
@@ -62,7 +62,7 @@ public:
         with default value of this argument the data written to the stream must
         be valid UTF-8, pass @c wxConvISO8859_1 to deal with arbitrary 8 bit data.
     */
-    wxStringOutputStream(wxString* pString = 0, wxMBConv& conv = wxConvUTF8);
+    explicit wxStringOutputStream(wxString* pString = NULL, wxMBConv& conv = wxConvUTF8);
 
     /**
         Returns the string containing all the data written to the stream so far.