diff --git a/cron/job_definition.py b/cron/job_definition.py
index 96560ef43b..44cc4a3a93 100644
--- a/cron/job_definition.py
+++ b/cron/job_definition.py
@@ -26,7 +26,7 @@ def merge_job_definition(local: Dict[str, Any], authored: Dict[str, Any]) -> Dic
     merged = {key: value for key, value in local.items() if key not in JOB_DEFINITION_FIELDS}
     merged.update((key, authored[key]) for key in JOB_DEFINITION_FIELDS if key in authored)
     merged["repeat"] = {
-        "completed": (local.get("repeat") or {}).get("completed", 0),
+        "completed": (local.get("repeat") or {}).get("completed") or 0,
         "times": (authored.get("repeat") or {}).get("times"),
     }
     if isinstance(merged.get("schedule"), str):
diff --git a/cron/jobs.py b/cron/jobs.py
index 734f3cb215..d34aa38db2 100644
--- a/cron/jobs.py
+++ b/cron/jobs.py
@@ -1973,7 +1973,7 @@ def _normalize_job_updates(job: Dict[str, Any], updates: Dict[str, Any]) -> None
             updates[key] = norm(updates[key])
     if "repeat" in updates:
         _rp = updates["repeat"]
-        completed = (job.get("repeat") or {}).get("completed", 0)
+        completed = (job.get("repeat") or {}).get("completed") or 0
         if isinstance(_rp, dict):
             _rp = dict(_rp)
             _rp["times"] = normalize_repeat_value(_rp.get("times"))
diff --git a/tests/cron/test_jobs.py b/tests/cron/test_jobs.py
index 434b8fabb0..98d58a445b 100644
--- a/tests/cron/test_jobs.py
+++ b/tests/cron/test_jobs.py
@@ -438,6 +438,25 @@ class TestJobCRUD:
         with pytest.raises(ValueError, match="Invalid repeat"):
             update_job(job["id"], {"repeat": "banana"})
 
+    def test_null_repeat_completed_counts_as_zero(self, tmp_cron_dir):
+        """A hand-edited "completed": null must not kill mark_job_run (None += 1) or be
+        carried forward by update_job."""
+        import json
+        from cron.jobs import JOBS_FILE, get_job, mark_job_run, update_job
+
+        job = create_job(prompt="t", schedule="every 1h", repeat=3)
+
+        def null_completed():
+            payload = json.loads(JOBS_FILE.read_text(encoding="utf-8"))
+            payload["jobs"][0]["repeat"]["completed"] = None
+            JOBS_FILE.write_text(json.dumps(payload), encoding="utf-8")
+
+        null_completed()
+        mark_job_run(job["id"], success=True)
+        assert get_job(job["id"])["repeat"]["completed"] == 1
+        null_completed()
+        assert update_job(job["id"], {"repeat": {"times": 5}})["repeat"]["completed"] == 0
+
     def test_oneshot_turned_recurring_becomes_forever(self, tmp_cron_dir):
         """A one-shot budget must not survive a schedule change to a recurring kind.