diff --git a/ml.cpp b/ml.cpp
index a913b88..7efee13 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -1941,7 +1941,8 @@ MlValue range(std::vector<MlValue> args, MlEnvironment &env) {
 
 	if (low >= high) return MlValue(result);
 
-	while (low < high) {
+	result.reserve(high.as_int() - low.as_int());
+	while (low < high) {	// use MlValue to keep number datatype
 		result.push_back(low);
 		low = low + MlValue(1l);
 	}