diff --git a/src/Data/CitationSource.php b/src/Data/CitationSource.php
index 335e8ad..3ee0bbe 100644
--- a/src/Data/CitationSource.php
+++ b/src/Data/CitationSource.php
@@ -23,7 +23,7 @@ public function __construct(
         public readonly int $startIndex,
         public readonly int $endIndex,
         public readonly string $uri,
-        public readonly string $license,
+        public readonly ?string $license,
     ) {}
 
     /**