Main Multisite Site Not Available

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.

Reproduce the issue in a WordPress multisite setup by enabling the service and checking the Portal Site choices. Trace how the main site is excluded from selection and verify that it can be chosen and that posts from sub-sites aggregate there.

Written by the indexing model from the issue text.