diff --git a/hc/api/tests/test_check_model.py b/hc/api/tests/test_check_model.py
index f8a2ead7..0c559a7d 100644
--- a/hc/api/tests/test_check_model.py
+++ b/hc/api/tests/test_check_model.py
@@ -9,11 +9,13 @@ class CheckModelTestCase(TestCase):
         check.save()
 
         for i in range(0, 6):
-            p = Ping(owner=check, ua="UA%d" % i)
+            p = Ping(pk=100 + i, owner=check, ua="UA%d" % i)
             p.save()
 
         check.prune_pings(keep_limit=3)
 
+        self.assertEqual(check.n_pings, 3)
+
         ua_set = set(Ping.objects.values_list("ua", flat=True))
         # UA0, UA1, UA2 should have been pruned--
         self.assertEqual(ua_set, set(["UA3", "UA4", "UA5"]))