diff --git a/nibabel/orientations.py b/nibabel/orientations.py
index b620fff02..f1cdd228b 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(map(list, 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):