diff --git a/src/Soap4me/CurlTrait.php b/src/Soap4me/CurlTrait.php
index 57d2529..15a993d 100644
--- a/src/Soap4me/CurlTrait.php
+++ b/src/Soap4me/CurlTrait.php
@@ -63,7 +63,7 @@ private function curl(string $url, array $data = [])
      */
     private function getHttpClient(): Client
     {
-        if (is_null($this->client)) {
+        if ($this->client === null) {
             $this->client = new Client([
                 'base_uri' => $this->baseUrl,
                 'timeout' => 5.0,