diff --git a/src/HttpMultipart/HttpFoundation/MultipartResponse.php b/src/HttpMultipart/HttpFoundation/MultipartResponse.php
index 783c8034..269b4e1e 100644
--- a/src/HttpMultipart/HttpFoundation/MultipartResponse.php
+++ b/src/HttpMultipart/HttpFoundation/MultipartResponse.php
@@ -20,7 +20,7 @@ class MultipartResponse extends ResourceResponse {
   /**
    * @var Response[]
    */
-  protected $parts;
+  protected $parts = [];
 
   /**
    * Constructor.
@@ -28,30 +28,37 @@ class MultipartResponse extends ResourceResponse {
   public function __construct(array $parts = NULL, $status = 200, $headers = [], $subtype = NULL) {
     parent::__construct(NULL, $status, $headers);
 
+    if ($parts) {
+      $this->setParts($parts);
+    }
+
     $this->subtype = $subtype ?: 'mixed';
     $this->boundary = md5(microtime());
+  }
 
-    if (NULL !== $parts) {
-      $this->setParts($parts);
+  /**
+   * {@inheritdoc}
+   */
+  public function prepare(Request $request) {
+    $this->headers->set('Content-Type', sprintf('multipart/%s; boundary="%s"', $this->subtype, $this->boundary));
+    $this->headers->set('Transfer-Encoding', 'chunked');
+
+    // Prepare the response content from the parts.
+    $parts = $this->getParts();
+
+    if ($parts) {
       $content = '';
-      foreach ($this->getParts() as $part) {
+
+      foreach ($parts as $part) {
         $content .= "--{$this->boundary}\r\n";
         $content .= "Content-Type: {$part->headers->get('Content-Type')}\r\n\r\n";
-        $content .= \Drupal::service('serializer')
-          ->serialize($part->getResponseData(), 'json');
+        $content .= $part->getContent();
         $content .= "\r\n";
       }
+
       $content .= "--{$this->boundary}--";
       $this->setContent($content);
     }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function prepare(Request $request) {
-    $this->headers->set('Content-Type', "multipart/{$this->subtype}; boundary=\"{$this->boundary}\"");
-    $this->headers->set('Transfer-Encoding', 'chunked');
 
     return parent::prepare($request);
   }
diff --git a/src/HttpMultipart/ResourceMultipartResponse.php b/src/HttpMultipart/ResourceMultipartResponse.php
index 7816e29b..5685d6ae 100644
--- a/src/HttpMultipart/ResourceMultipartResponse.php
+++ b/src/HttpMultipart/ResourceMultipartResponse.php
@@ -26,24 +26,10 @@ public function prepare(Request $request) {
   public function sendContent() {
     // This fixes the "Malformed encoding found in chunked-encoding"
     // error message in curl and makes possible to get the correct response body.
-    $size = $this->getSize();
+    $size = strlen($this->getContent());
     echo "$size\r\n";
-    parent::sendContent();
-  }
 
-  /**
-   * Returns the length of all the parts in the response body.
-   *
-   * @return int
-   */
-  protected function getSize() {
-    $size = 0;
-    foreach ($this->parts as $part) {
-      $content = $part->getContent();
-      $output = "--{$this->boundary}" . "{$part->headers}" . $content;
-      $size += strlen($output);
-    }
-    return $size;
+    parent::sendContent();
   }
 
 }