diff --git a/dlibrary/dlibrary.py b/dlibrary/dlibrary.py
index c246cc9..cf8b69b 100755
--- a/dlibrary/dlibrary.py
+++ b/dlibrary/dlibrary.py
@@ -606,10 +606,12 @@ def unique_hierarchical_prefix_numbering(entries, start_point=0):
 def alphabetic_numbering(entries, start_point):
     alphabetized = {}
     for entry in entries:
-        ending = nfc(entry.stem)[start_point:]
+        ending = nfc(entry.stem)[start_point:].strip(' -_()')
         if len(ending) > 1:
             return None
         index = 0 if ending == '' else ord(ending.lower()) - ord('a') + 1
+        if index < 0 or index > 26:
+            return None
         if (index,) in alphabetized:
             return None
         alphabetized[(index,)] = [entry]