diff --git a/src/activity_core/issue_sink.py b/src/activity_core/issue_sink.py
index f9dccd4..73f8f60 100644
--- a/src/activity_core/issue_sink.py
+++ b/src/activity_core/issue_sink.py
@@ -66,7 +66,11 @@ class IssueCoreRestSink(IssueSink):
             "due_in_days": task_spec.due_in_days,
             "source_type": task_spec.source_type,
             "source_id": task_spec.source_id,
-            "triggering_event_id": task_spec.triggering_event_id,
+            "triggering_event_id": (
+                str(task_spec.triggering_event_id)
+                if task_spec.triggering_event_id is not None
+                else None
+            ),
             "activity_definition_id": task_spec.activity_definition_id,
         }
         resp = httpx.post(