diff --git a/dlibrary/dlibrary.py b/dlibrary/dlibrary.py
index d0c17dc..14adba3 100755
--- a/dlibrary/dlibrary.py
+++ b/dlibrary/dlibrary.py
@@ -816,7 +816,10 @@ def unique_hierarchical_prefix_numbering(entries, start_point=0):
                     ents_idx = numbering.pop(idx)
                     debug(f'Index {idx} has multiple entries')
                     longest = max(ents_idx, key=lambda e: len(nname(e)))
-                    next_layer_start = pos + NUMBER_REGEX.match(nname(longest)[pos:]).end()
+                    next_match = NUMBER_REGEX.match(nname(longest)[pos:])
+                    if not next_match:
+                        return None
+                    next_layer_start = pos + next_match.end()
                     sub_numbering = unique_hierarchical_prefix_numbering(ents_idx, start_point=next_layer_start) or alphabetic_numbering(ents_idx, next_layer_start)
                     if not sub_numbering:
                         return None