diff --git a/radicale-auth-ldap/__init__.py b/radicale-auth-ldap/__init__.py
index 3705b1e..23e207f 100644
--- a/radicale-auth-ldap/__init__.py
+++ b/radicale-auth-ldap/__init__.py
@@ -23,10 +23,10 @@ PLUGIN_CONFIG_SCHEMA = {"auth": {
         "value": None,
         "type": str},
     "ldap_access_group_filter": {
-        "value": None,
+        "value": "None",
         "type": str},
     "ldap_access_group_attribute": {
-        "value": None,
+        "value": "None",
         "type": str}}}
 
 class Auth(BaseAuth):
@@ -61,7 +61,7 @@ class Auth(BaseAuth):
             logger.warning("LDAP: binddn credentials are invalid")
             return ""
 
-        if (not accessGroupFilter == None) and (not accessGroupAttribute == None):
+        if (not accessGroupFilter == "None") and (not accessGroupAttribute == "None"):
             logger.debug("LDAP access group filter: %s" % accessGroupFilter)
             logger.debug("LDAP access group attribute: %s" % accessGroupAttribute)