diff --git a/lib/Coroutine.php b/lib/Coroutine.php
index d4195bfe..a283a3df 100644
--- a/lib/Coroutine.php
+++ b/lib/Coroutine.php
@@ -88,7 +88,7 @@ public function __construct(\Generator $generator)
          * @psalm-suppress MissingClosureParamType
          * @psalm-suppress MissingClosureReturnType
          */
-        $onResolve = function (?\Throwable $e = null, $v) use ($generator, &$onResolve) {
+        $onResolve = function (?\Throwable $e, $v) use ($generator, &$onResolve) {
             /** @var bool $immediate Used to control iterative coroutine continuation. */
             static $immediate = true;