diff --git a/ml.cpp b/ml.cpp
index 343e326..d82debc 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -1879,6 +1879,7 @@ MlValue map_list(std::vector<MlValue> args, MlEnvironment &env) {
 		throw MlError(MlValue("map_list", map_list), env, args.size() > 2 ? TOO_MANY_ARGS : TOO_FEW_ARGS);
 
 	std::vector<MlValue> result, list = args[1].as_list(), tmp;
+	result.reserve(list.size());
 	for (const auto & item : list) {
 		tmp.push_back(item);
 		result.push_back(args[0].apply(tmp, env));