diff --git a/src/Future.php b/src/Future.php
index 3c5cffb..09eb56c 100644
--- a/src/Future.php
+++ b/src/Future.php
@@ -135,6 +135,7 @@ final class Future
             }
 
             try {
+                /** @var T $value */
                 $state->complete($onComplete($value));
             } catch (\Throwable $exception) {
                 $state->error($exception);
diff --git a/src/Internal/Cancellable.php b/src/Internal/Cancellable.php
index 240406d..cdf5008 100644
--- a/src/Internal/Cancellable.php
+++ b/src/Internal/Cancellable.php
@@ -18,8 +18,7 @@ final class Cancellable implements Cancellation
     /** @var callable[] */
     private array $callbacks = [];
 
-    /** @var \Throwable|null */
-    private ?\Throwable $exception = null;
+    private ?CancelledException $exception = null;
 
     public function cancel(?\Throwable $previous = null): void
     {
diff --git a/src/Internal/FutureState.php b/src/Internal/FutureState.php
index 43d9e1f..a5da429 100644
--- a/src/Internal/FutureState.php
+++ b/src/Internal/FutureState.php
@@ -21,7 +21,7 @@ final class FutureState
     private bool $handled = false;
 
     /**
-     * @var array<string, callable(?\Throwable, ?T, string): void>
+     * @var array<string, \Closure(?\Throwable, ?T, string): void>
      */
     private array $callbacks = [];
 
@@ -45,12 +45,12 @@ final class FutureState
      *
      * The callback is invoked directly from the event loop context, so suspension within the callback is not possible.
      *
-     * @param callable(?\Throwable, ?T, string): void $callback Callback invoked on error / successful completion of
+     * @param \Closure(?\Throwable, ?T, string): void $callback Callback invoked on error / successful completion of
      * the future.
      *
      * @return string Identifier that can be used to cancel interest for this future.
      */
-    public function subscribe(callable $callback): string
+    public function subscribe(\Closure $callback): string
     {
         $id = self::$nextId++;
 
diff --git a/src/Internal/functions.php b/src/Internal/functions.php
index 3678fa8..8668129 100644
--- a/src/Internal/functions.php
+++ b/src/Internal/functions.php
@@ -5,8 +5,8 @@ namespace Amp\Internal;
 /**
  * Formats a stacktrace obtained via `debug_backtrace()`.
  *
- * @param array<array{file?: string, line: int, type?: string, class: string, function: string}> $trace Output of
- *     `debug_backtrace()`.
+ * @param list<array{args?:list<mixed>, class?: class-string, file: string, function: string, line: int, object?: object, type?: string}> $trace
+ * Output of `debug_backtrace()`.
  *
  * @return string Formatted stacktrace.
  *
@@ -22,7 +22,7 @@ function formatStacktrace(array $trace): string
             $line .= "{$e['file']}:{$e['line']} ";
         }
 
-        if (isset($e["type"])) {
+        if (isset($e["class"], $e["type"])) {
             $line .= $e["class"] . $e["type"];
         }