diff --git a/Classes/EventListener/Order/Create/Number.php b/Classes/EventListener/Order/Create/Number.php
index e3f8d07c..6379f0bf 100644
--- a/Classes/EventListener/Order/Create/Number.php
+++ b/Classes/EventListener/Order/Create/Number.php
@@ -43,9 +43,9 @@ protected function generateNumber(NumberGeneratorEventInterface $event): string
         $number = sprintf($format, $numberInRegistryWithOffset);
 
         return implode('', [
-            is_string($this->options['prefix']) ? $this->options['prefix'] : '',
+            is_string($this->options['prefix'] ?? null) ? $this->options['prefix'] : '',
             $number,
-            is_string($this->options['suffix']) ? $this->options['suffix'] : '',
+            is_string($this->options['suffix'] ?? null) ? $this->options['suffix'] : '',
         ]);
     }
 }