diff --git a/src/blackboard/operations.py b/src/blackboard/operations.py
index 05394db..c9cb446 100644
--- a/src/blackboard/operations.py
+++ b/src/blackboard/operations.py
@@ -329,7 +329,7 @@ class Blackboard:
     # Observation
     # ===================================================================
 
-    def add_observation(self, task_id: str, observer: str, body: str,
+    def add_observation(self, task_id: Optional[str], observer: str, body: str,
                         severity: str = "info") -> int:
         conn = self._conn()
         try: