diff --git a/UnitPay.php b/UnitPay.php
index 1f56bfe..cabcec9 100644
--- a/UnitPay.php
+++ b/UnitPay.php
@@ -310,7 +310,7 @@ public function api($method, array $params = [])
         $requestUrl = $this->apiUrl . '?' . http_build_query([
                 'method' => $method,
                 'params' => $params,
-            ], null, '&', PHP_QUERY_RFC3986);
+            ], "", '&', PHP_QUERY_RFC3986);
 
         $response = json_decode(file_get_contents($requestUrl));
         if (!is_object($response)) {