diff --git a/include/wx/string.h b/include/wx/string.h
index 0562d9a915..2bc8b54afd 100644
--- a/include/wx/string.h
+++ b/include/wx/string.h
@@ -1078,7 +1078,7 @@ public:
   static wxString FormatV(const wxChar *pszFormat, va_list argptr);
 
     // returns the highest possible memory allocation for encoding
-  static size_t WorstEncodingCase(size_t len, wxMBConv& conv);
+  static size_t WorstEncodingCase(size_t len, const wxMBConv& conv);
 
   // raw access to string memory
     // ensure that string has space for at least nLen characters
diff --git a/src/common/string.cpp b/src/common/string.cpp
index 51acab0189..f6b388e45b 100644
--- a/src/common/string.cpp
+++ b/src/common/string.cpp
@@ -988,7 +988,7 @@ int STRINGCLASS::compare(size_t nStart, size_t nLen,
 // common conversion routines
 // ---------------------------------------------------------------------------
 
-size_t wxString::WorstEncodingCase(size_t len, wxMBConv& WXUNUSED(conv))
+size_t wxString::WorstEncodingCase(size_t len, const wxMBConv& WXUNUSED(conv))
 {
     //Worst case for UTF7
     return len * 5;