diff --git a/src/sentry/testutils/helpers/datetime.py b/src/sentry/testutils/helpers/datetime.py
index c68d1db640b2c4..da1af95d8c5c51 100644
--- a/src/sentry/testutils/helpers/datetime.py
+++ b/src/sentry/testutils/helpers/datetime.py
@@ -8,7 +8,7 @@
 __all__ = ["iso_format", "before_now", "timestamp_format"]
 
 
-def iso_format(date):
+def iso_format(date: datetime) -> str:
     return date.isoformat()[:19]
 
 
diff --git a/tests/snuba/api/serializers/test_group.py b/tests/snuba/api/serializers/test_group.py
index 90fe986d0cffe0..955cf14a5bcc4c 100644
--- a/tests/snuba/api/serializers/test_group.py
+++ b/tests/snuba/api/serializers/test_group.py
@@ -408,7 +408,8 @@ def test_seen_stats(self):
         # result is rounded down to nearest second
         assert iso_format(result["lastSeen"]) == iso_format(self.min_ago)
         assert iso_format(result["firstSeen"]) == iso_format(group_env.first_seen)
-        assert iso_format(group_env2.first_seen) > iso_format(group_env.first_seen)
+        assert group_env2.first_seen is not None
+        assert group_env2.first_seen > group_env.first_seen
         assert result["userCount"] == 3
 
         result = serialize(