diff --git a/src/VoucherService/TokenManager/Pattern.php b/src/VoucherService/TokenManager/Pattern.php
index 5d09e6af..572a8638 100644
--- a/src/VoucherService/TokenManager/Pattern.php
+++ b/src/VoucherService/TokenManager/Pattern.php
@@ -257,16 +257,16 @@ public function getFinalTokenLength(): int
     {
         $separatorCount = $this->configuration->getSeparatorCount();
         $separator = $this->configuration->getSeparator();
-        $prefix = $this->configuration->getPrefix();
+        $prefix = (string)$this->configuration->getPrefix();
         if (!empty($separator)) {
             if (!empty($prefix)) {
-                return strlen($this->configuration->getPrefix()) + 1 + (int) floor($this->configuration->getLength() / $separatorCount) + $this->configuration->getLength();
+                return strlen($prefix) + 1 + (int) floor($this->configuration->getLength() / $separatorCount) + $this->configuration->getLength();
             }
 
             return (int) floor($this->configuration->getLength() / $separatorCount) + $this->configuration->getLength();
         }
 
-        return strlen($this->configuration->getPrefix()) + $this->configuration->getLength();
+        return strlen($prefix) + $this->configuration->getLength();
     }
 
     /**