diff --git a/tests/domain/abri/test_slots.py b/tests/domain/abri/test_slots.py
index acc0a65fd..fa5283691 100644
--- a/tests/domain/abri/test_slots.py
+++ b/tests/domain/abri/test_slots.py
@@ -36,3 +36,12 @@ def test_a_confirmed_time_at_or_after_midday_books_the_afternoon_slot(
 
     # Assert
     assert slot == "PM"
+
+
+@pytest.mark.parametrize("confirmed_time", ["half nine", "25:00", "9.30am"])
+def test_an_unparseable_confirmed_time_fails_loudly_rather_than_booking_all_day(
+    confirmed_time: str,
+) -> None:
+    # Act / Assert
+    with pytest.raises(ValueError):
+        slot_for_confirmed_time(confirmed_time)