diff --git a/src/Redmine/Api/Issue.php b/src/Redmine/Api/Issue.php
index 298fb4a0..895c612e 100644
--- a/src/Redmine/Api/Issue.php
+++ b/src/Redmine/Api/Issue.php
@@ -2,6 +2,7 @@
 
 namespace Redmine\Api;
 
+use Redmine\Serializer\JsonSerializer;
 use Redmine\Serializer\PathSerializer;
 use Redmine\Serializer\XmlSerializer;
 
@@ -266,13 +267,15 @@ public function attach($id, array $attachment)
      */
     public function attachMany($id, array $attachments)
     {
-        $request = [];
-        $request['issue'] = [
+        $params = [
             'id' => $id,
             'uploads' => $attachments,
         ];
 
-        return $this->put('/issues/'.$id.'.json', json_encode($request));
+        return $this->put(
+            '/issues/'.$id.'.json',
+            JsonSerializer::createFromArray(['issue' => $params])->getEncoded()
+        );
     }
 
     /**
diff --git a/src/Redmine/Api/IssueRelation.php b/src/Redmine/Api/IssueRelation.php
index 1574b9ba..31b319a5 100644
--- a/src/Redmine/Api/IssueRelation.php
+++ b/src/Redmine/Api/IssueRelation.php
@@ -85,9 +85,10 @@ public function create($issueId, array $params = [])
 
         $params = $this->sanitizeParams($defaults, $params);
 
-        $params = json_encode(['relation' => $params]);
-
-        $response = $this->post('/issues/'.urlencode($issueId).'/relations.json', $params);
+        $response = $this->post(
+            '/issues/'.urlencode($issueId).'/relations.json',
+            JsonSerializer::createFromArray(['relation' => $params])->getEncoded()
+        );
 
         return JsonSerializer::createFromString($response)->getNormalized();
     }