diff --git a/src/builtin/list.rs b/src/builtin/list.rs
index af2e602..477308b 100644
--- a/src/builtin/list.rs
+++ b/src/builtin/list.rs
@@ -8,6 +8,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
     f.insert(
         "length".to_owned(),
         Box::new(|args, _| {
+            max_args!(args, 1);
             let len = match arg!(args, 0, "list") {
                 Value::List(v, _) => Number::from(v.len()),
                 _ => Number::from(1),