diff --git a/src/core/query_transformation.h b/src/core/query_transformation.h
index a8332b8fb..36d2096af 100644
--- a/src/core/query_transformation.h
+++ b/src/core/query_transformation.h
@@ -19,7 +19,7 @@ namespace details
 {
 
 // TODO: use std::unary_function<const std::string&, std::string>
-class query_transformation_function : public std::unary_function<std::string, std::string>
+class query_transformation_function : public std::unary_function<std::string const&, std::string>
 {
 public:
     virtual result_type operator()(argument_type a) const = 0;