diff --git a/src/Postmark/Transport.php b/src/Postmark/Transport.php
index 34be046..1c912bd 100644
--- a/src/Postmark/Transport.php
+++ b/src/Postmark/Transport.php
@@ -96,7 +96,7 @@ public function send(Swift_Mime_SimpleMessage $message, &$failedRecipients = nul
 
 		$success = $response->getStatusCode() === 200;
 
-		if ($responseEvent = $this->_eventDispatcher->createResponseEvent($this, $response->getBody()->getContents(), $success)) {
+		if ($responseEvent = $this->_eventDispatcher->createResponseEvent($this, $response->getBody()->__toString(), $success)) {
 			$this->_eventDispatcher->dispatchEvent($responseEvent, 'responseReceived');
 		}
 
diff --git a/tests/TransportGuzzleStreamConsumptionTest.php b/tests/TransportGuzzleStreamConsumptionTest.php
new file mode 100644
index 0000000..beaf513
--- /dev/null
+++ b/tests/TransportGuzzleStreamConsumptionTest.php
@@ -0,0 +1,123 @@
+<?php
+
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\TestCase;
+use Postmark\ThrowExceptionOnFailurePlugin;
+use Postmark\Transport;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+class TransportGuzzleStreamConsumptionTest extends TestCase
+{
+
+    public function testSendWithoutMiddleware()
+    {
+        $message = new Swift_Message();
+
+        $transport = new TransportGuzzleStreamConsumptionTestPostmarkTransportStubNoMiddleware([
+            new Response(422, [], 'Some error from server'),
+        ]);
+        $transport->registerPlugin(new ThrowExceptionOnFailurePlugin());
+
+        $exception = null;
+        try {
+            $transport->send($message);
+        } catch (Swift_TransportException $exception) {
+            // Deliberately empty
+        }
+
+        $this->assertNotNull($exception);
+        $this->assertInstanceOf(Swift_TransportException::class, $exception);
+        $this->assertSame('Some error from server', $exception->getMessage());
+    }
+
+    public function testSendWithMiddleware()
+    {
+        $message = new Swift_Message();
+
+        $transport = new TransportGuzzleStreamConsumptionTestPostmarkTransportStubWithConsumingMiddleware([
+            new Response(422, [], 'Some error from server'),
+        ]);
+        $transport->registerPlugin(new ThrowExceptionOnFailurePlugin());
+
+        $exception = null;
+        try {
+            $transport->send($message);
+        } catch (Swift_TransportException $exception) {
+            // Deliberately empty
+        }
+
+        $this->assertNotNull($exception);
+        $this->assertInstanceOf(Swift_TransportException::class, $exception);
+
+        // This would fail if \Postmark\Transport::send would use
+        // getBody->getContents() instead of getBody->__toString()
+        $this->assertSame('Some error from server', $exception->getMessage());
+    }
+}
+
+class TransportGuzzleStreamConsumptionTestPostmarkTransportStubNoMiddleware extends Transport
+{
+    protected $client;
+
+    public function __construct(array $responses = [])
+    {
+        parent::__construct('TESTING_SERVER');
+
+        $this->client = $this->mockGuzzle($responses);
+    }
+
+    protected function getHttpClient()
+    {
+        return $this->client;
+    }
+
+    private function mockGuzzle(array $responses)
+    {
+        $stack = HandlerStack::create(new MockHandler($responses));
+
+        return new Client(['handler' => $stack]);
+    }
+}
+
+class TransportGuzzleStreamConsumptionTestPostmarkTransportStubWithConsumingMiddleware extends Transport
+{
+    protected $client;
+
+    public function __construct(array $responses = [])
+    {
+        parent::__construct('TESTING_SERVER');
+
+        $this->client = $this->mockGuzzle($responses);
+    }
+
+    protected function getHttpClient()
+    {
+        return $this->client;
+    }
+
+    private function mockGuzzle(array $responses)
+    {
+        $stack = HandlerStack::create(new MockHandler($responses));
+        $stack->push(
+            function (callable $handler) {
+                return function ($request, array $options) use ($handler) {
+                    return $handler($request, $options)->then(
+                        function (Response $response) {
+                            // Pretend to do something with $response, like logging
+                            $response->getBody()->__toString();
+
+                            return $response;
+                        }
+                    );
+                };
+            }
+        );
+
+        return new Client(['handler' => $stack]);
+    }
+}
+