diff --git a/test_timestamp.py b/test_timestamp.py
index ba6ec59..fadceca 100644
--- a/test_timestamp.py
+++ b/test_timestamp.py
@@ -1,23 +1,23 @@
 import datetime
 import pytest  # type: ignore
 
 from timestamp import datetime_to_utc_timestamp, utc_timestamp_to_datetime
 
 
-def test_datetime_to_utc_timestamp():
+def test_datetime_to_utc_timestamp() -> None:
     dt = datetime.datetime(2019, 3, 17, 13, 23, 28, tzinfo=datetime.timezone.utc)
     assert datetime_to_utc_timestamp(dt) == 1552829008
 
 @pytest.mark.parametrize('dt', [
     datetime.datetime.now(),
     datetime.datetime.utcnow(),
     datetime.datetime.now(datetime.timezone(datetime.timedelta(hours=1))),
     datetime.datetime(2019, 3, 17, 13, 23, 28, 251638, tzinfo=datetime.timezone.utc)
 ])
-def test_datetime_to_utc_timestamp_invalid_timezone(dt):
+def test_datetime_to_utc_timestamp_invalid_timezone(dt: datetime.datetime) -> None:
     with pytest.raises(AssertionError):
         datetime_to_utc_timestamp(dt)
 
-def test_utc_timestamp_to_datetime():
+def test_utc_timestamp_to_datetime() -> None:
     dt = datetime.datetime(2019, 3, 17, 13, 23, 28, tzinfo=datetime.timezone.utc)
     assert utc_timestamp_to_datetime(1552829008) == dt