IOException:IDX20807:无法从以下位置检索文档：“System.String”。HttpResponseMessage：

我正在使用 Asp.Net Core 3.1 并通过 Challenge 方法，调用外部端点进行身份验证

public async Task<IActionResult> Challenge(string provider, string returnUrl,string userName)
        {
           
                var props = new AuthenticationProperties
                {
                    RedirectUri = Url.Action(nameof(Callback)),
                    Items =
                    {
                        { "returnUrl", returnUrl },
                        { "scheme", provider },
                    },
                    Parameters =
                    {
                        { OidcConstants.AuthorizeRequest.LoginHint, userName }
                    }
                };

                var result = Challenge(props, provider);
                return result;
            }

错误：

An unhandled exception occurred while processing the request.
IOException: IDX20807: Unable to retrieve document from: 'System.String'. HttpResponseMessage: 'System.Net.Http.HttpResponseMessage', HttpResponseMessage.Content: 'System.String'.
Microsoft.IdentityModel.Protocols.HttpDocumentRetriever.GetDocumentAsync(string address, CancellationToken cancel)

InvalidOperationException: IDX20803: Unable to obtain configuration from: 'System.String'.
Microsoft.IdentityModel.Protocols.ConfigurationManager<T>.GetConfigurationAsync(CancellationToken cancel)

感谢，任何快速反应。