diff --git a/src/caoscrawler/crawl.py b/src/caoscrawler/crawl.py
index 3b8ba58d7fee9a30ed55665ca95334745627ba8b..00be826ef2fba2156a2741eac2c9674fc9875b26 100644
--- a/src/caoscrawler/crawl.py
+++ b/src/caoscrawler/crawl.py
@@ -555,7 +555,7 @@ class Crawler(object):
         return False
 
     @staticmethod
-    def create_flat_list(ent_list: list[db.Entity], flat: Optional[set[db.Entity]]):
+    def create_flat_list(ent_list: list[db.Entity], flat: Optional[set[db.Entity]] = None):
         """
         Recursively adds entities and all their properties contained in ent_list to
         the output set flat.