diff --git a/src/WildcardPermission.php b/src/WildcardPermission.php
index a4287187..a27b8d67 100644
--- a/src/WildcardPermission.php
+++ b/src/WildcardPermission.php
@@ -43,7 +43,7 @@ public function getIndex(): array
     protected function buildIndex(array $index, array $parts, string $permission): array
     {
         if (empty($parts)) {
-            $index[null] = true;
+            $index[''] = true;
 
             return $index;
         }