Henrik Lindström helped me out with the answer.

This is because it triggers Find to referens and trying to serielize the whole PageData object and this wount work.

The way to solve it is to use IncludeField.

Start by creating extension methods to PageData like this:

        public static string HitId(this PageData page)
        {
            return SearchClient.Instance.Conventions.IdConvention.GetId(page);
        }

        public static string HitType(this PageData page)
        {
            return SearchClient.Instance.Conventions.TypeNameConvention.GetTypeName(page.GetType());
        }

Then in your class that defines what to or not to index (if you have one of those) add this:

            SearchClient.Instance.Conventions.ForInstancesOf<PageData>().IncludeField(p => p.HitId());
            SearchClient.Instance.Conventions.ForInstancesOf<PageData>().IncludeField(p => p.HitType());

Reindex all and then use the new property like this:

HitId = h.HitId(),
HitType = h.HitType()

It works great!

Thanks again Henrik!