diff --git a/lib/HttpRequestGraphQL.php b/lib/HttpRequestGraphQL.php
index 708f334..84fa2f6 100644
--- a/lib/HttpRequestGraphQL.php
+++ b/lib/HttpRequestGraphQL.php
@@ -45,13 +45,15 @@ protected static function prepareRequest($httpHeaders = array(), $data = array()
         }
 
         self::$httpHeaders = $httpHeaders;
+        self::$httpHeaders['Content-type'] = 'application/json';
 
         if (is_array($variables)) {
             self::$postDataGraphQL = json_encode(['query' => $data, 'variables' => $variables]);
-            self::$httpHeaders['Content-type'] = 'application/json';
         } else {
-            self::$httpHeaders['Content-type'] = 'application/graphql';
+            self::$postDataGraphQL = json_encode(['query' => $data]);
         }
+
+        self::$httpHeaders['Content-Length'] = strlen(self::$postDataGraphQL);
     }
 
     /**