diff --git a/src/QentaCEE/Stdlib/FingerprintOrder.php b/src/QentaCEE/Stdlib/FingerprintOrder.php
index 69c17b9..7dd7822 100644
--- a/src/QentaCEE/Stdlib/FingerprintOrder.php
+++ b/src/QentaCEE/Stdlib/FingerprintOrder.php
@@ -132,7 +132,7 @@ public function __toString()
      * @param int|string $mOffset
      * @param int|string $mValue
      */
-    public function offsetSet($mOffset, $mValue)
+    public function offsetSet($mOffset, $mValue) : void
     {
         if (!$mOffset) {
             $this->_fingeprintOrder[] = trim($mValue);