diff --git a/include/external/clara.h b/include/external/clara.h
index 369c1b76..a999a37b 100644
--- a/include/external/clara.h
+++ b/include/external/clara.h
@@ -396,9 +396,12 @@ namespace Clara {
         inline void convertInto( std::string const& _source, std::string& _dest ) {
             _dest = _source;
         }
+        char toLowerCh(char c) {
+            return static_cast<char>( ::tolower( c ) );
+        }
         inline void convertInto( std::string const& _source, bool& _dest ) {
             std::string sourceLC = _source;
-            std::transform( sourceLC.begin(), sourceLC.end(), sourceLC.begin(), ::tolower );
+            std::transform( sourceLC.begin(), sourceLC.end(), sourceLC.begin(), toLowerCh );
             if( sourceLC == "y" || sourceLC == "1" || sourceLC == "true" || sourceLC == "yes" || sourceLC == "on" )
                 _dest = true;
             else if( sourceLC == "n" || sourceLC == "0" || sourceLC == "false" || sourceLC == "no" || sourceLC == "off" )