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DotNetOpenAuth with MVC 4.0
var request = _openid.CreateRequest(openIdUrl);
  var fetch = new FetchRequest();
  fetch.Attributes.AddRequired(WellKnownAttributes.Contact.Email);
  fetch.Attributes.AddRequired(WellKnownAttributes.Name.First);
  fetch.Attributes.AddRequired(WellKnownAttributes.Name.Last);
  request.AddExtension(fetch);
  //return RedirectToAction("Login");
  return request.RedirectingResponse.AsActionResult(); // <-- This is the line throwing the error

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
        <dependentAssembly>
            <assemblyIdentity name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" />
            <bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0" />
        </dependentAssembly>
        <dependentAssembly>
            <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
            <bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" />
        </dependentAssembly>
        <dependentAssembly>
            <assemblyIdentity name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" />
            <bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0" />
            <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
        </dependentAssembly>
    </assemblyBinding>
    <legacyHMACWarning enabled="0" />
</runtime>

using DotNetOpenAuth.OpenId.Extensions.AttributeExchange;
using DotNetOpenAuth.OpenId.Extensions;