diff --git a/ndfilters/_tests/test_trimmed_mean.py b/ndfilters/_tests/test_trimmed_mean.py
index 989c0b2..dfb1e03 100644
--- a/ndfilters/_tests/test_trimmed_mean.py
+++ b/ndfilters/_tests/test_trimmed_mean.py
@@ -20,6 +20,7 @@
 @pytest.mark.parametrize(
     argnames="axis",
     argvalues=[
+        None,
         0,
         -1,
         (0,),