diff --git a/tests/test_client_waiter.py b/tests/test_client_waiter.py
index 0c3d27e2..d503b72c 100644
--- a/tests/test_client_waiter.py
+++ b/tests/test_client_waiter.py
@@ -44,7 +44,7 @@ def test_flow():
         return waiter
 
     test_flow()
-    assert mock_waiter.wait.called_once_with("JobExists")
+    mock_waiter.wait.assert_called_once_with("JobExists")
 
 
 @mock_ec2
@@ -66,4 +66,4 @@ def test_flow():
         return waiter
 
     test_flow()
-    assert mock_waiter.wait.called_once_with("instance_exists")
+    mock_waiter.wait.assert_called_once_with("instance_exists")