diff --git a/src/Rule.php b/src/Rule.php
index ab1c2a8..97dad0e 100644
--- a/src/Rule.php
+++ b/src/Rule.php
@@ -248,7 +248,7 @@ public function triggerFailureEvent(Facts $facts): array
         ];
     }
 
-    public function interpretRules(array $conditions = null): string
+    public function interpretRules(?array $conditions = null): string
     {
         if ($conditions === null) {
             $conditions = $this->conditions;