diff --git a/lib/HttpRequestGraphQL.php b/lib/HttpRequestGraphQL.php
index fe6106c..84fa2f6 100644
--- a/lib/HttpRequestGraphQL.php
+++ b/lib/HttpRequestGraphQL.php
@@ -52,6 +52,8 @@ protected static function prepareRequest($httpHeaders = array(), $data = array()
         } else {
             self::$postDataGraphQL = json_encode(['query' => $data]);
         }
+
+        self::$httpHeaders['Content-Length'] = strlen(self::$postDataGraphQL);
     }
 
     /**