diff --git a/src/Psl/Iter/last_key_opt.php b/src/Psl/Iter/last_key_opt.php
index 25551ad8..4e32faa8 100644
--- a/src/Psl/Iter/last_key_opt.php
+++ b/src/Psl/Iter/last_key_opt.php
@@ -17,7 +17,7 @@
  *
  * @return Option<Tk>
  */
-function last_key_opt(iterable $iterable)
+function last_key_opt(iterable $iterable): Option
 {
     $last = Option::none();
     foreach ($iterable as $k => $_) {