diff --git a/packages/app/resource/locales/en_US/translation.json b/packages/app/resource/locales/en_US/translation.json
index 5f219aaf5d7..08ffe8daa91 100644
--- a/packages/app/resource/locales/en_US/translation.json
+++ b/packages/app/resource/locales/en_US/translation.json
@@ -984,6 +984,7 @@
     "application_already_installed": "Application already installed.",
     "email_address_could_not_be_used": "This email address could not be used. (Make sure the allowed email address)",
     "user_id_is_not_available":"This User ID is not available.",
+    "username_should_not_be_null":"Username should not be null. Please check Authentication Mechanism Settings on admin page",
     "email_address_is_already_registered":"This email address is already registered.",
     "can_not_register_maximum_number_of_users":"Can not register more than the maximum number of users.",
     "failed_to_register":"Failed to register.",
diff --git a/packages/app/resource/locales/ja_JP/translation.json b/packages/app/resource/locales/ja_JP/translation.json
index 7c7a45c60cf..5c1ef7ae86c 100644
--- a/packages/app/resource/locales/ja_JP/translation.json
+++ b/packages/app/resource/locales/ja_JP/translation.json
@@ -977,6 +977,7 @@
     "application_already_installed": "アプリケーションのインストールが完了しました。",
     "email_address_could_not_be_used":"このメールアドレスは使用できません。(許可されたメールアドレスを確認してください。)",
     "user_id_is_not_available":"このユーザーIDは使用できません。",
+    "username_should_not_be_null":"Username が null になっています 管理画面の認証機構設定にて設定の確認をしてください",
     "email_address_is_already_registered":"このメールアドレスは既に登録されています。",
     "can_not_register_maximum_number_of_users":"ユーザー数が上限を超えたため登録できません。",
     "failed_to_register":"登録に失敗しました。",
diff --git a/packages/app/resource/locales/zh_CN/translation.json b/packages/app/resource/locales/zh_CN/translation.json
index 6e394a959fc..68d4a445cab 100644
--- a/packages/app/resource/locales/zh_CN/translation.json
+++ b/packages/app/resource/locales/zh_CN/translation.json
@@ -986,7 +986,8 @@
 		"aws_sttings_required": "使用此功能所需的AWS设置。请询问管理员。",
 		"application_already_installed": "应用程序已安装。",
 		"email_address_could_not_be_used": "无法使用此电子邮件地址。（确保允许的电子邮件地址）",
-		"user_id_is_not_available": "此用户ID不可用。",
+    "user_id_is_not_available": "此用户ID不可用。",
+    "username_should_not_be_null":"用户名不应为空。请检查管理页面上的身份验证机制设置",
 		"email_address_is_already_registered": "此电子邮件地址已注册。",
 		"can_not_register_maximum_number_of_users": "注册的用户数不能超过最大值。",
 		"failed_to_register": "注册失败。",
diff --git a/packages/app/src/server/models/errors.ts b/packages/app/src/server/models/errors.ts
index eab693c2cfe..3019b26644e 100644
--- a/packages/app/src/server/models/errors.ts
+++ b/packages/app/src/server/models/errors.ts
@@ -10,3 +10,9 @@ export class PathAlreadyExistsError extends ExtensibleCustomError {
   }
 
 }
+
+
+/*
+* User Authentication
+*/
+export class NullUsernameToBeRegisteredError extends ExtensibleCustomError {}
diff --git a/packages/app/src/server/models/external-account.js b/packages/app/src/server/models/external-account.js
index c98d0e49d50..828f4197de8 100644
--- a/packages/app/src/server/models/external-account.js
+++ b/packages/app/src/server/models/external-account.js
@@ -1,5 +1,6 @@
 // disable no-return-await for model functions
 /* eslint-disable no-return-await */
+import { NullUsernameToBeRegisteredError } from '~/server/models/errors';
 
 const debug = require('debug')('growi:models:external-account');
 const mongoose = require('mongoose');
@@ -104,6 +105,10 @@ class ExternalAccount {
           return account;
         }
 
+        if (usernameToBeRegistered == null) {
+          throw new NullUsernameToBeRegisteredError('username_should_not_be_null');
+        }
+
         const User = ExternalAccount.crowi.model('User');
 
         let promise = User.findOne({ username: usernameToBeRegistered });
diff --git a/packages/app/src/server/routes/login-passport.js b/packages/app/src/server/routes/login-passport.js
index c84b159f21e..79a6fe557fa 100644
--- a/packages/app/src/server/routes/login-passport.js
+++ b/packages/app/src/server/routes/login-passport.js
@@ -1,3 +1,4 @@
+import { NullUsernameToBeRegisteredError } from '~/server/models/errors';
 import loggerFactory from '~/utils/logger';
 
 /* eslint-disable no-use-before-define */
@@ -109,9 +110,11 @@ module.exports = function(crowi, app) {
     const attrMapUsername = passportService.getLdapAttrNameMappedToUsername();
     const attrMapName = passportService.getLdapAttrNameMappedToName();
     const attrMapMail = passportService.getLdapAttrNameMappedToMail();
-    const usernameToBeRegistered = ldapAccountInfo[attrMapUsername];
+    // const usernameToBeRegistered = ldapAccountInfo[attrMapUsername];
+    const usernameToBeRegistered = null;
     const nameToBeRegistered = ldapAccountInfo[attrMapName];
     const mailToBeRegistered = ldapAccountInfo[attrMapMail];
+
     const userInfo = {
       id: ldapAccountId,
       username: usernameToBeRegistered,
@@ -578,7 +581,11 @@ module.exports = function(crowi, app) {
     }
     catch (err) {
       /* eslint-disable no-else-return */
-      if (err.name === 'DuplicatedUsernameException') {
+      if (err instanceof NullUsernameToBeRegisteredError) {
+        req.flash('warningMessage', req.t(`message.${err.message}`));
+        return;
+      }
+      else if (err.name === 'DuplicatedUsernameException') {
         if (isSameEmailTreatedAsIdenticalUser || isSameUsernameTreatedAsIdenticalUser) {
           // associate to existing user
           debug(`ExternalAccount '${userInfo.username}' will be created and bound to the exisiting User account`);