diff --git a/src/sentry/monitors/serializers.py b/src/sentry/monitors/serializers.py
index 83b69a12f0e2d3..e75aa3fd111e05 100644
--- a/src/sentry/monitors/serializers.py
+++ b/src/sentry/monitors/serializers.py
@@ -274,7 +274,7 @@ class MonitorCheckInSerializerResponse(MonitorCheckInSerializerResponseOptional)
     id: str
     environment: str
     status: str
-    duration: int
+    duration: int | None
     dateCreated: datetime
     dateAdded: datetime
     dateUpdated: datetime