Signing out doesn't work with secondary reply URL

I have an application that is set up to authenticate against a WAAD tenant. The application configuration in the Azure Management Portal specifies two reply URLs for the application:

http://<myapp>.cloudapp.net/

http://localhost:1915/

(Note: the first URL also matches my Realm, if that matters)

In development, my application is set up to use the latter reply URL, and single sign-in works fine. However, when I try to perform a sign-out request:

var config = FederatedAuthentication.FederationConfiguration;

string wtrealm = config.WsFederationConfiguration.Realm;
string wreply = config.WsFederationConfiguration.Reply;

var signoutRequestMessage = new SignOutRequestMessage(new Uri(config.WsFederationConfiguration.Issuer));
signoutRequestMessage.Parameters.Add("wreply", wreply);
signoutRequestMessage.Parameters.Add("wtrealm", wtrealm);

FederatedAuthentication.SessionAuthenticationModule.SignOut();

context.HttpContext.Response.Redirect(signoutRequestMessage.WriteQueryString());

 the request fails with an ACS error saying the reply URL is invalid. If I switch to the first reply URL, sign-out works too -- but of course, not in the development environment. What's up?