diff --git a/lib/CancellationTokenSource.php b/lib/CancellationTokenSource.php
index 2432dd2..fcfa925 100644
--- a/lib/CancellationTokenSource.php
+++ b/lib/CancellationTokenSource.php
@@ -2,9 +2,6 @@
 
 namespace Amp;
 
-use Revolt\EventLoop\Loop;
-use function Revolt\EventLoop\defer;
-
 /**
  * A cancellation token source provides a mechanism to cancel operations.
  *
@@ -40,72 +37,13 @@ use function Revolt\EventLoop\defer;
  */
 final class CancellationTokenSource
 {
+    private Internal\CancellableToken $source;
     private CancellationToken $token;
 
-    /** @var callable|null */
-    private $onCancel;
-
     public function __construct()
     {
-        $onCancel = &$this->onCancel;
-
-        $this->token = new class($onCancel) implements CancellationToken {
-            private string $nextId = "a";
-
-            /** @var callable[] */
-            private array $callbacks = [];
-
-            /** @var \Throwable|null */
-            private ?\Throwable $exception = null;
-
-            /**
-             * @param callable|null $onCancel
-             * @param-out callable $onCancel
-             */
-            public function __construct(?callable &$onCancel)
-            {
-                $onCancel = function (\Throwable $exception): void {
-                    $this->exception = $exception;
-
-                    $callbacks = $this->callbacks;
-                    $this->callbacks = [];
-
-                    foreach ($callbacks as $callback) {
-                        Loop::queue($callback, $this->exception);
-                    }
-                };
-            }
-
-            public function subscribe(callable $callback): string
-            {
-                $id = $this->nextId++;
-
-                if ($this->exception) {
-                    Loop::queue($callback, $this->exception);
-                } else {
-                    $this->callbacks[$id] = $callback;
-                }
-
-                return $id;
-            }
-
-            public function unsubscribe(string $id): void
-            {
-                unset($this->callbacks[$id]);
-            }
-
-            public function isRequested(): bool
-            {
-                return isset($this->exception);
-            }
-
-            public function throwIfRequested(): void
-            {
-                if (isset($this->exception)) {
-                    throw $this->exception;
-                }
-            }
-        };
+        $this->source = new Internal\CancellableToken;
+        $this->token = new Internal\WrappedCancellationToken($this->source);
     }
 
     public function getToken(): CancellationToken
@@ -116,14 +54,8 @@ final class CancellationTokenSource
     /**
      * @param \Throwable|null $previous Exception to be used as the previous exception to CancelledException.
      */
-    public function cancel(\Throwable $previous = null): void
+    public function cancel(?\Throwable $previous = null): void
     {
-        if ($this->onCancel === null) {
-            return;
-        }
-
-        $onCancel = $this->onCancel;
-        $this->onCancel = null;
-        $onCancel(new CancelledException($previous));
+        $this->source->cancel($previous);
     }
 }
diff --git a/lib/CancelledException.php b/lib/CancelledException.php
index 25f8c40..7cc1da4 100644
--- a/lib/CancelledException.php
+++ b/lib/CancelledException.php
@@ -10,7 +10,7 @@ namespace Amp;
  */
 class CancelledException extends \Exception
 {
-    public function __construct(\Throwable $previous = null)
+    public function __construct(?\Throwable $previous = null)
     {
         parent::__construct("The operation was cancelled", 0, $previous);
     }
diff --git a/lib/Internal/CancellableToken.php b/lib/Internal/CancellableToken.php
new file mode 100644
index 0000000..79a080a
--- /dev/null
+++ b/lib/Internal/CancellableToken.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Amp\Internal;
+
+use Amp\CancellationToken;
+use Amp\CancelledException;
+use Revolt\EventLoop\Loop;
+
+/**
+ * Cancellation Token with public cancellation method.
+ *
+ * @internal
+ */
+final class CancellableToken implements CancellationToken
+{
+    private string $nextId = "a";
+
+    /** @var callable[] */
+    private array $callbacks = [];
+
+    /** @var \Throwable|null */
+    private ?\Throwable $exception = null;
+
+    public function cancel(?\Throwable $previous = null): void
+    {
+        if (isset($this->exception)) {
+            return;
+        }
+
+        $this->exception = new CancelledException($previous);
+
+        $callbacks = $this->callbacks;
+        $this->callbacks = [];
+
+        foreach ($callbacks as $callback) {
+            Loop::queue($callback, $this->exception);
+        }
+    }
+
+    public function subscribe(callable $callback): string
+    {
+        $id = $this->nextId++;
+
+        if ($this->exception) {
+            Loop::queue($callback, $this->exception);
+        } else {
+            $this->callbacks[$id] = $callback;
+        }
+
+        return $id;
+    }
+
+    public function unsubscribe(string $id): void
+    {
+        unset($this->callbacks[$id]);
+    }
+
+    public function isRequested(): bool
+    {
+        return isset($this->exception);
+    }
+
+    public function throwIfRequested(): void
+    {
+        if (isset($this->exception)) {
+            throw $this->exception;
+        }
+    }
+}
diff --git a/lib/Internal/WrappedCancellationToken.php b/lib/Internal/WrappedCancellationToken.php
new file mode 100644
index 0000000..a7aa8a2
--- /dev/null
+++ b/lib/Internal/WrappedCancellationToken.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Amp\Internal;
+
+use Amp\CancellationToken;
+
+/**
+ * @internal
+ */
+final class WrappedCancellationToken implements CancellationToken
+{
+    public function __construct(
+        private CancellationToken $token
+    ) {
+    }
+
+    public function subscribe(callable $callback): string
+    {
+        return $this->token->subscribe($callback);
+    }
+
+    public function unsubscribe(string $id): void
+    {
+        $this->token->unsubscribe($id);
+    }
+
+    public function isRequested(): bool
+    {
+        return $this->token->isRequested();
+    }
+
+    public function throwIfRequested(): void
+    {
+        $this->token->throwIfRequested();
+    }
+}
diff --git a/lib/TimeoutCancellationToken.php b/lib/TimeoutCancellationToken.php
index 53b8956..e94b425 100644
--- a/lib/TimeoutCancellationToken.php
+++ b/lib/TimeoutCancellationToken.php
@@ -19,13 +19,19 @@ final class TimeoutCancellationToken implements CancellationToken
      */
     public function __construct(float $timeout, string $message = "Operation timed out")
     {
-        $source = new CancellationTokenSource;
-        $this->token = $source->getToken();
+        $this->token = $source = new Internal\CancellableToken;
+
+        $trace = null; // Defined in case assertions are disabled.
+        \assert($trace = \debug_backtrace());
 
-        $trace = \debug_backtrace(\DEBUG_BACKTRACE_IGNORE_ARGS);
         $this->watcher = Loop::delay($timeout, static function () use ($source, $message, $trace): void {
-            $trace = Internal\formatStacktrace($trace);
-            $source->cancel(new TimeoutException("$message\r\nTimeoutCancellationToken was created here:\r\n$trace"));
+            if ($trace) {
+                $message .= \sprintf("\r\n%s was created here: %s", self::class, Internal\formatStacktrace($trace));
+            } else {
+                $message .= \sprintf(" (Enable assertions for a backtrace of the %s creation)", self::class);
+            }
+
+            $source->cancel(new TimeoutException($message));
         });
 
         Loop::unreference($this->watcher);
diff --git a/test/TimeoutCancellationTokenTest.php b/test/TimeoutCancellationTokenTest.php
index c2f8432..9df14a4 100644
--- a/test/TimeoutCancellationTokenTest.php
+++ b/test/TimeoutCancellationTokenTest.php
@@ -26,8 +26,11 @@ class TimeoutCancellationTokenTest extends AsyncTestCase
             self::assertInstanceOf(TimeoutException::class, $exception->getPrevious());
 
             $message = $exception->getPrevious()->getMessage();
-            self::assertStringContainsString('TimeoutCancellationToken was created here', $message);
-            self::assertStringContainsString('TimeoutCancellationTokenTest.php:' . $line, $message);
+
+            if ((int)ini_get('zend.assertions') > 0) {
+                self::assertStringContainsString('TimeoutCancellationToken was created here', $message);
+                self::assertStringContainsString('TimeoutCancellationTokenTest.php:' . $line, $message);
+            }
         }
     }