I'm using Asp.Net Core 3.1 and thru Challenge method, calling an external endpoint to authenticate

```
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;
            }

```

Error:

```
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)

```

Appreciate, any quick response.