diff --git a/src/sisl/geom/nanoribbon.py b/src/sisl/geom/nanoribbon.py
index 1bb95f2525..2d6db6e2d4 100644
--- a/src/sisl/geom/nanoribbon.py
+++ b/src/sisl/geom/nanoribbon.py
@@ -294,6 +294,8 @@ class _heteroribbon_section(CompositeGeometrySection):
         The bond length of the ribbon.
     kind: {'armchair', 'zigzag'}
         The kind of ribbon that this section should be.
+    vacuum :
+        minimum separation in transverse direction
     shift_quantum: bool, optional
         Whether the implementation will assist avoiding lone atoms (< 2 neighbours).