diff --git a/nanobot/cron/service.py b/nanobot/cron/service.py
index b9833307c..ce0df35b4 100644
--- a/nanobot/cron/service.py
+++ b/nanobot/cron/service.py
@@ -80,12 +80,14 @@ def _validate_schedule_for_add(schedule: CronSchedule) -> None:
             raise ValueError("cron schedule requires a non-empty 'expr'")
         try:
             from croniter import croniter
+
             croniter(schedule.expr)
-        except Exception as e:
-            raise ValueError(f"invalid cron expression '{schedule.expr}': {e}") from None
+        except Exception as exc:
+            raise ValueError(f"invalid cron expression '{schedule.expr}': {exc}") from None
         if schedule.tz:
             try:
                 from zoneinfo import ZoneInfo
+
                 ZoneInfo(schedule.tz)
             except Exception:
                 raise ValueError(f"unknown timezone '{schedule.tz}'") from None
diff --git a/tests/cron/test_cron_service.py b/tests/cron/test_cron_service.py
index f91bf2b89..5e18c6450 100644
--- a/tests/cron/test_cron_service.py
+++ b/tests/cron/test_cron_service.py
@@ -141,6 +141,33 @@ def test_add_job_accepts_valid_timezone(tmp_path) -> None:
     assert job.state.next_run_at_ms is not None
 
 
+@pytest.mark.parametrize("expr", [None, "", "   "])
+def test_add_job_rejects_missing_cron_expression(tmp_path, expr: str | None) -> None:
+    service = CronService(tmp_path / "cron" / "jobs.json")
+
+    with pytest.raises(ValueError, match="requires a non-empty 'expr'"):
+        service.add_job(
+            name="missing expression",
+            schedule=CronSchedule(kind="cron", expr=expr),
+            message="hello",
+        )
+
+    assert service.list_jobs(include_disabled=True) == []
+
+
+def test_add_job_rejects_invalid_cron_expression_before_persisting(tmp_path) -> None:
+    service = CronService(tmp_path / "cron" / "jobs.json")
+
+    with pytest.raises(ValueError, match="invalid cron expression"):
+        service.add_job(
+            name="bad expression",
+            schedule=CronSchedule(kind="cron", expr="not a cron expression"),
+            message="hello",
+        )
+
+    assert service.list_jobs(include_disabled=True) == []
+
+
 def test_write_run_record_uses_cron_runs_dir(tmp_path) -> None:
     service = CronService(tmp_path / "cron" / "jobs.json")