Main Multisite Site Not Available

[PillarTy]

When enabling this service, it would appear as the "Portal Site" cannot be the main site in a multisite setup. I can select any other site, but not the main site.

In our use case, we would like to aggregate all posts from every sub-site to the main site in the network. We are using this as an intranet in which every internal department as their own sub-site. Aggregating blogs entries to the main site is essential.