--- a/extensions/spellcheck/hunspell/glue/mozHunspell.h
+++ b/extensions/spellcheck/hunspell/glue/mozHunspell.h
@@ -94,17 +94,17 @@ public:
 
   mozHunspell();
 
   nsresult Init();
 
   void LoadDictionaryList(bool aNotifyChildProcesses);
 
   // helper method for converting a word to the charset of the dictionary
-  nsresult ConvertCharset(const char16_t* aStr, char ** aDst);
+  nsresult ConvertCharset(const char16_t* aStr, std::string* aDst);
 
   NS_DECL_NSIMEMORYREPORTER
 
 protected:
   virtual ~mozHunspell();
 
   nsCOMPtr<mozIPersonalDictionary> mPersonalDictionary;
   nsCOMPtr<nsIUnicodeEncoder>      mEncoder;