diff --git a/src/Logger.php b/src/Logger.php
index 930684664..00a24cd52 100644
--- a/src/Logger.php
+++ b/src/Logger.php
@@ -102,7 +102,7 @@ public function logSoapCall($client, $operation)
         );
 
         $this->writeMessage(
-            static::getSoapCallResponseMessage($client),
+            static::getSoapCallResponseMessage($client) ?? '',
             ['operation' => $operation, 'type' => self::TYPE_RESPONSE]
         );
     }