diff --git a/nibabel/orientations.py b/nibabel/orientations.py
index 12e414def..b620fff02 100644
--- a/nibabel/orientations.py
+++ b/nibabel/orientations.py
@@ -322,7 +322,7 @@ def axcodes2ornt(axcodes, labels=None):
            [ 2.,  1.]])
     """
     labels = list(zip('LPI', 'RAS')) if labels is None else labels
-    allowed_labels = sum([list(L) for L in labels], []) + [None]
+    allowed_labels = sum((list(L) for L in labels), []) + [None]
     if len(allowed_labels) != len(set(allowed_labels)):
         raise ValueError(f'Duplicate labels in {allowed_labels}')
     if not set(axcodes).issubset(allowed_labels):
diff --git a/nibabel/tests/test_volumeutils.py b/nibabel/tests/test_volumeutils.py
index 9d321f07e..1bd44cbd0 100644
--- a/nibabel/tests/test_volumeutils.py
+++ b/nibabel/tests/test_volumeutils.py
@@ -607,7 +607,7 @@ def test_a2f_nanpos():
 
 def test_a2f_bad_scaling():
     # Test that pathological scalers raise an error
-    NUMERICAL_TYPES = sum([sctypes[key] for key in ['int', 'uint', 'float', 'complex']], [])
+    NUMERICAL_TYPES = sum((sctypes[key] for key in ['int', 'uint', 'float', 'complex']), [])
     for in_type, out_type, slope, inter in itertools.product(
         NUMERICAL_TYPES,
         NUMERICAL_TYPES,