diff --git a/Lib/ldap/modlist.py b/Lib/ldap/modlist.py
index a853500..e1dfe44 100644
--- a/Lib/ldap/modlist.py
+++ b/Lib/ldap/modlist.py
@@ -51,9 +51,9 @@ def modifyModlist(
   modlist = []
   attrtype_lower_map = {}
   for a in old_entry.keys():
-    attrtype_lower_map[str.lower(a)]=a
+    attrtype_lower_map[a.lower()]=a
   for attrtype in new_entry.keys():
-    attrtype_lower = str.lower(attrtype)
+    attrtype_lower = attrtype.lower()
     if attrtype_lower in ignore_attr_types:
       # This attribute type is ignored
       continue