Autofact not respecting InstancePerRequest

I'm using Autofac on an MVC project with Web API.

I'm registering all controllers:

builder.RegisterControllers(assembly);
builder.RegisterApiControllers(assembly);

and

DependencyResolver.SetResolver(new AutofacDependencyResolver(container));
GlobalConfiguration.Configuration.DependencyResolver = new AutofacWebApiDependencyResolver(container);

I'm also using OWIN, and I'm registering a UserManager per OWIN request:

app.CreatePerOwinContext(() => AutofacDependencyResolver.Current.GetService<UserManager>());

The UserManager needs an instance of DbContext, which is configured to be instantiated once per request:

builder.RegisterType<DbContext>()
       .AsSelf()
       .InstancePerRequest();

Here is the problem:

When accessing a Web API controller, aDbContext is created to satisfy the CreatePerOwinContext call. Then, in my controller, I inject another DbContext, and I expected the instance injected in the controller to be the same created in the CreatePerOwinContext call, but it is not! If I access an MVC controller, everything works as expected.

I think Autofac is using two different resolvers, and not sharing instances between them. How can I solve this?