Using Autofac For DI Into WCF Service Hosted In Web Application?

I'm having trouble injecting services dependencies into my WCF service using Autofac 1.4.5. I've read and followed the Autofac wiki page on WcfIntegration but my debugging shows me that my WCF service is created by the System.ServiceModel.Dispatcher.InstanceBehavior.GetInstance() method and not by the AutofacWebServiceHostFactory. What am I doing wrong?

I've set up my ajax.svc file to look like the one in the example for use with WebHttpBinding:

<%@ ServiceHost Language="C#" Debug="true"
Service="Generic.Frontend.Web.Ajax, Generic.Frontend.Web"
Factory="Autofac.Integration.Wcf.AutofacWebServiceHostFactory,
Autofac.Integration.Wcf" %>

My WCF service class Ajax is defined like this:

namespace Generic.Frontend.Web
{
[ServiceContract]
[AspNetCompatibilityRequirements(
RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class Ajax
{
public MapWebService MapWebService { get; set;}.....

The service already works fine but I can't get the Autofac bits (read: creation/injection) to work.

Removing the default constructor unfortunately leads to the following exception:

System.InvalidOperationException: The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host.