diff --git a/ipalib/plugins/config.py b/ipalib/plugins/config.py
index 848b41e7b..e7b6e791c 100644
--- a/ipalib/plugins/config.py
+++ b/ipalib/plugins/config.py
@@ -128,6 +128,7 @@ class config(LDAPObject):
             cli_name='maxusername',
             label=_('Maximum username length'),
             minvalue=1,
+            maxvalue=255,
         ),
         IA5Str('ipahomesrootdir',
             cli_name='homedirectory',