diff --git a/src/Gateways/Test/TestGateway.php b/src/Gateways/Test/TestGateway.php
index 7e70278..2976ec5 100644
--- a/src/Gateways/Test/TestGateway.php
+++ b/src/Gateways/Test/TestGateway.php
@@ -85,7 +85,7 @@ public function purchaseUri(): string
         $url_parts['query'] = http_build_query($params);
 
         return $url_parts['scheme'].'://'.$url_parts['host']
-            .(strlen($url_parts['port']) ? ':'.$url_parts['port'] : '')
+            .(isset($url_parts['port']) && strlen($url_parts['port']) ? ':'.$url_parts['port'] : '')
             .$url_parts['path']
             .(strlen($url_parts['query']) ? '?'.$url_parts['query'] : '');
     }