diff --git a/include/fintamath/parser/Parser.hpp b/include/fintamath/parser/Parser.hpp
index 6997e82d3..fd7b09f2f 100644
--- a/include/fintamath/parser/Parser.hpp
+++ b/include/fintamath/parser/Parser.hpp
@@ -21,7 +21,7 @@ class Parser {
   using Comparator = std::function<bool(Args...)>;
 
   template <typename Return, typename... Args>
-  using Map = std::multimap<std::string, Function<Return, Args...>>;
+  using Map = std::unordered_multimap<std::string, Function<Return, Args...>>;
 
   template <typename Return, typename... Args>
   using Vector = std::vector<Function<Return, Args...>>;