diff --git a/NicoSitePlugin2/Api.cs b/NicoSitePlugin2/Api.cs
index 2a54e17f..6e51dd88 100644
--- a/NicoSitePlugin2/Api.cs
+++ b/NicoSitePlugin2/Api.cs
@@ -30,9 +30,7 @@ public static async Task<UserInfo> GetUserInfo(IDataSource server, CookieContain
             var res = await server.GetAsync(url, cc);
             var obj = JsonConvert.DeserializeObject<NicoSitePlugin2.Low.UserInfo.RootObject>(res);
             if (obj.Data.Length == 0)
-            {
-                throw new ArgumentException("指定されたuserIdは存在しない:" + userId);
-            }
+                return null;
             var data = obj.Data[0];
             var userInfo = new UserInfo
             {
diff --git a/NicoSitePlugin2/TestCommentProvider.cs b/NicoSitePlugin2/TestCommentProvider.cs
index 3eb732de..22fc1562 100644
--- a/NicoSitePlugin2/TestCommentProvider.cs
+++ b/NicoSitePlugin2/TestCommentProvider.cs
@@ -417,17 +417,13 @@ private async Task ProcessChatMessageAsync(Chat.IChatMessage message)
                             {
                                 _chatProvider?.Disconnect();
                             }
-                            string username;
+                            string username = null;
                             if (IsRawUserId(chat.UserId) && chat.UserId != SystemUserId && _siteOptions.IsAutoGetUsername)
                             {
                                 var userInfo = await Api.GetUserInfo(_server, _cc, chat.UserId);
-                                username = userInfo.Nickname;
+                                username = userInfo?.Nickname;
                                 user.Name = Common.MessagePartFactory.CreateMessageItems(username);
                             }
-                            else
-                            {
-                                username = null;
-                            }
                             if (_siteOptions.IsAutoSetNickname)
                             {
                                 var nick = SitePluginCommon.Utils.ExtractNickname(chat.Content);