diff --git a/airflow/models/serialized_dag.py b/airflow/models/serialized_dag.py
index 71a3de413ceba..59d2567188462 100644
--- a/airflow/models/serialized_dag.py
+++ b/airflow/models/serialized_dag.py
@@ -280,7 +280,7 @@ def get_last_updated_datetime(cls, dag_id: str, session: Session = None) -> Opti
 
     @classmethod
     @provide_session
-    def get_max_last_updated_datetime(cls, session: Session = None) -> datetime:
+    def get_max_last_updated_datetime(cls, session: Session = None) -> Optional[datetime]:
         """
         Get the maximum date when any DAG was last updated in serialized_dag table
 
@@ -291,7 +291,7 @@ def get_max_last_updated_datetime(cls, session: Session = None) -> datetime:
 
     @classmethod
     @provide_session
-    def get_latest_version_hash(cls, dag_id: str, session: Session = None) -> str:
+    def get_latest_version_hash(cls, dag_id: str, session: Session = None) -> Optional[str]:
         """
         Get the latest DAG version for a given DAG ID.
 
@@ -299,7 +299,7 @@ def get_latest_version_hash(cls, dag_id: str, session: Session = None) -> str:
         :type dag_id: str
         :param session: ORM Session
         :type session: Session
-        :return: DAG Hash
+        :return: DAG Hash, or None if the DAG is not found
         :rtype: str | None
         """
         return session.query(cls.dag_hash).filter(cls.dag_id == dag_id).scalar()
diff --git a/airflow/www/views.py b/airflow/www/views.py
index 009bcac87b187..a1e61721f28ae 100644
--- a/airflow/www/views.py
+++ b/airflow/www/views.py
@@ -4093,7 +4093,8 @@ def list(self):
         title = "DAG Dependencies"
 
         if timezone.utcnow() > self.last_refresh + self.refresh_interval:
-            if SerializedDagModel.get_max_last_updated_datetime() > self.last_refresh:
+            max_last_updated = SerializedDagModel.get_max_last_updated_datetime()
+            if max_last_updated is None or max_last_updated > self.last_refresh:
                 self._calculate_graph()
             self.last_refresh = timezone.utcnow()