diff --git a/src/allmydata/util/idlib.py b/src/allmydata/util/idlib.py
index a972b22e7..8432f5801 100644
--- a/src/allmydata/util/idlib.py
+++ b/src/allmydata/util/idlib.py
@@ -161,9 +161,9 @@ s8 = init_s8()
 # string tells whether the final character is reasonable.
 def init_s5():
     s5 = []
-    add_check_array(chars, s5)
-    for lenmod5 in (1, 2, 3, 4,):
-        add_check_array(get_trailing_chars_without_lsbs(4-lenmod5), s5)
+    add_check_array(get_trailing_chars_without_lsbs(0), s5)
+    for lenmod5 in [1,2,3,4]:
+        add_check_array(get_trailing_chars_without_lsbs(5-lenmod5), s5)
     return tuple(s5)
 s5 = init_s5()