title: IOException: IDX20807: Unable to retrieve document from: 'System.String'. HttpResponseMessage:
body: <p>I'm using Asp.Net Core 3.1 and thru Challenge method, calling an external endpoint to authenticate</p>
  <pre><code>public async Task&lt;IActionResult&gt; Challenge(string provider, string returnUrl,string userName)
          {
             
                  var props = new AuthenticationProperties
                  {
                      RedirectUri = Url.Action(nameof(Callback)),
                      Items =
                      {
                          { &quot;returnUrl&quot;, returnUrl },
                          { &quot;scheme&quot;, provider },
                      },
                      Parameters =
                      {
                          { OidcConstants.AuthorizeRequest.LoginHint, userName }
                      }
                  };
  
                  var result = Challenge(props, provider);
                  return result;
              }
  </code></pre>
  <p>Error:</p>
  <pre><code>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&lt;T&gt;.GetConfigurationAsync(CancellationToken cancel)
  </code></pre>
  <p>Appreciate, any quick response.</p>