Replay the static call to Zend_Json with a static call to Magento JsonConverter.

 - This is only in place for this one use case and should not be used for other times with converting json

Author: dmanners

lib/internal/Magento/Framework/DataObject.php

@@ -325,19 +325,21 @@ public function convertToXml(
      * Convert object data to JSON
      *
      * @param array $keys array of required keys
-     * @return string
+     * @return bool|string
+     * @throws \InvalidArgumentException
      */
     public function toJson(array $keys = [])
     {
         $data = $this->toArray($keys);
-        return \Zend_Json::encode($data);
+        return \Magento\Framework\Serialize\JsonConverter::convert($data);
     }
 
     /**
      * The "__" style wrapper for toJson
      *
-     * @param  array $keys
-     * @return string
+     * @param array $keys
+     * @return bool|string
+     * @throws \InvalidArgumentException
      */
     public function convertToJson(array $keys = [])
     {

lib/internal/Magento/Framework/Serialize/JsonConverter.php

@@ -0,0 +1,25 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Framework\Serialize;
+
+/**
+ * Used to convert \Magento\Framework\DataObject to Json
+ *
+ * @deprecated @see \Magento\Framework\Serialize\Serializer\Json::serialize
+ */
+class JsonConverter
+{
+    /**
+     * @param $data
+     * @return bool|string
+     * @throws \InvalidArgumentException
+     */
+    public static function convert($data)
+    {
+        $serializer = new \Magento\Framework\Serialize\Serializer\Json();
+        return $serializer->serialize($data);
+    }
+}