diff --git a/lib/Worker/Internal/TaskSuccess.php b/lib/Worker/Internal/TaskSuccess.php
index 531fbfc..962cdba 100644
--- a/lib/Worker/Internal/TaskSuccess.php
+++ b/lib/Worker/Internal/TaskSuccess.php
@@ -2,6 +2,8 @@
 
 namespace Amp\Parallel\Worker\Internal;
 
+use Amp\Failure;
+use Amp\Parallel\Worker\Task;
 use Amp\Promise;
 use Amp\Success;
 
@@ -19,6 +21,13 @@ final class TaskSuccess extends TaskResult
 
     public function promise(): Promise
     {
+        if ($this->result instanceof \__PHP_Incomplete_Class) {
+            return new Failure(new \Error(\sprintf(
+                "Class instances returned from %s::run() must be autoloadable by the Composer autoloader",
+                Task::class
+            )));
+        }
+
         return new Success($this->result);
     }
 }
diff --git a/lib/Worker/TaskRunner.php b/lib/Worker/TaskRunner.php
index d86d982..98ed26d 100644
--- a/lib/Worker/TaskRunner.php
+++ b/lib/Worker/TaskRunner.php
@@ -4,6 +4,7 @@ namespace Amp\Parallel\Worker;
 
 use Amp\Coroutine;
 use Amp\Parallel\Sync\Channel;
+use Amp\Parallel\Sync\SerializationException;
 use Amp\Promise;
 use function Amp\call;
 
@@ -50,7 +51,12 @@ final class TaskRunner
 
             $job = null; // Free memory from last job.
 
-            yield $this->channel->send($result);
+            try {
+                yield $this->channel->send($result);
+            } catch (SerializationException $exception) {
+                // Could not serialize task result.
+                yield $this->channel->send(new Internal\TaskFailure($result->getId(), $exception));
+            }
 
             $result = null; // Free memory from last result.
 
diff --git a/test/Worker/AbstractWorkerTest.php b/test/Worker/AbstractWorkerTest.php
index 0630c9c..ba5f7d3 100644
--- a/test/Worker/AbstractWorkerTest.php
+++ b/test/Worker/AbstractWorkerTest.php
@@ -7,6 +7,7 @@ use Amp\Parallel\Sync\SerializationException;
 use Amp\Parallel\Worker\Environment;
 use Amp\Parallel\Worker\Task;
 use Amp\Parallel\Worker\TaskError;
+use Amp\Parallel\Worker\TaskException;
 use Amp\Parallel\Worker\WorkerException;
 use Amp\PHPUnit\TestCase;
 
@@ -203,7 +204,7 @@ abstract class AbstractWorkerTest extends TestCase
                     {
                     }
                 });
-                $this->fail("Tasks that cannot be autoloaded should throw an exception");
+                $this->fail("Tasks that cannot be serialized should throw an exception");
             } catch (SerializationException $exception) {
                 $this->assertSame(0, \strpos($exception->getMessage(), "The given data cannot be sent because it is not serializable"));
             }
@@ -212,6 +213,38 @@ abstract class AbstractWorkerTest extends TestCase
         });
     }
 
+    public function testUnserializableResult()
+    {
+        Loop::run(function () {
+            $worker = $this->createWorker();
+
+            try {
+                yield $worker->enqueue(new UnserializableResultTask);
+                $this->fail("Tasks results that cannot be serialized should throw an exception");
+            } catch (TaskException $exception) {
+                $this->assertSame(0, \strpos($exception->getMessage(), "Uncaught Amp\Parallel\Sync\SerializationException in worker"));
+            }
+
+            yield $worker->shutdown();
+        });
+    }
+
+    public function testNonAutoloadableResult()
+    {
+        Loop::run(function () {
+            $worker = $this->createWorker();
+
+            try {
+                yield $worker->enqueue(new NonAutoloadableResultTask);
+                $this->fail("Tasks results that cannot be autoloaded should throw an exception");
+            } catch (\Error $exception) {
+                $this->assertSame(0, \strpos($exception->getMessage(), "Class instances returned from Amp\Parallel\Worker\Task::run() must be autoloadable by the Composer autoloader"));
+            }
+
+            yield $worker->shutdown();
+        });
+    }
+
     public function testUnserializableTaskFollowedByValidTask()
     {
         Loop::run(function () {
diff --git a/test/Worker/NonAutoloadableResultTask.php b/test/Worker/NonAutoloadableResultTask.php
new file mode 100644
index 0000000..d32c4e7
--- /dev/null
+++ b/test/Worker/NonAutoloadableResultTask.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace Amp\Parallel\Test\Worker;
+
+use Amp\Parallel\Worker\Environment;
+use Amp\Parallel\Worker\Task;
+
+class NonAutoloadableResultTask implements Task
+{
+    public function run(Environment $environment)
+    {
+        require __DIR__ . "/non-autoloadable-class.php";
+        return new NonAutoloadableClass;
+    }
+}
diff --git a/test/Worker/UnserializableResultTask.php b/test/Worker/UnserializableResultTask.php
new file mode 100644
index 0000000..09d0857
--- /dev/null
+++ b/test/Worker/UnserializableResultTask.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace Amp\Parallel\Test\Worker;
+
+use Amp\Parallel\Worker\Environment;
+use Amp\Parallel\Worker\Task;
+
+class UnserializableResultTask implements Task
+{
+    public function run(Environment $environment)
+    {
+        return function () {}; // Anonymous functions are not serializable.
+    }
+}
diff --git a/test/Worker/non-autoloadable-class.php b/test/Worker/non-autoloadable-class.php
new file mode 100644
index 0000000..edf4003
--- /dev/null
+++ b/test/Worker/non-autoloadable-class.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace Amp\Parallel\Test\Worker;
+
+class NonAutoloadableClass
+{
+}