diff --git a/ctapipe/image/reducer.py b/ctapipe/image/reducer.py
index f7e63734d53..30ad1eda574 100644
--- a/ctapipe/image/reducer.py
+++ b/ctapipe/image/reducer.py
@@ -99,8 +99,8 @@ class NullDataVolumeReducer(DataVolumeReducer):
     """
 
     def select_pixels(self, waveforms, tel_id=None, selected_gain_channel=None):
-        mask = waveforms != 0
-        return mask
+        n_pixels = waveforms.shape[-2]
+        return np.ones(n_pixels, dtype=bool)
 
 
 class TailCutsDataVolumeReducer(DataVolumeReducer):
diff --git a/ctapipe/io/tests/test_hdf5.py b/ctapipe/io/tests/test_hdf5.py
index 1ffa6c97750..75f8a19b206 100644
--- a/ctapipe/io/tests/test_hdf5.py
+++ b/ctapipe/io/tests/test_hdf5.py
@@ -312,7 +312,8 @@ class WithUnits(Container):
     with tables.open_file(path, "r") as f:
         assert f.root.data.units.attrs["CTAFIELD_0_UNIT"] == "m**-1"
         assert f.root.data.units.attrs["CTAFIELD_1_UNIT"] == "s"
-        assert f.root.data.units.attrs["CTAFIELD_2_UNIT"] == "cm**-2.g"
+        # order of the units does not matter
+        assert f.root.data.units.attrs["CTAFIELD_2_UNIT"] in {"cm**-2.g", "g.cm**-2"}
 
 
 def test_write_containers(tmp_path):
diff --git a/docs/changes/2337.bug.rst b/docs/changes/2337.bugfix.rst
similarity index 100%
rename from docs/changes/2337.bug.rst
rename to docs/changes/2337.bugfix.rst
diff --git a/docs/changes/2340.bugfix.rst b/docs/changes/2340.bugfix.rst
new file mode 100644
index 00000000000..2b37cbba9d6
--- /dev/null
+++ b/docs/changes/2340.bugfix.rst
@@ -0,0 +1 @@
+Fix shape of mask returned by ``NullDataVolumeReducer``.