One can manually edit and add System Type - Table Fields. When adding secondary sources (and possibly other places), Data Type is required.
Added after initial report by Tom Ghinder: All Active fields should also require datatype (and associated fields) as they also pose a problem when adding secondary sources (Target Field Page in Target Design)
The page mentioned should be enhanced to ensure the above fields are populated in all cases.
This is the error that occurs when adding or validating a secondary source:
This is where the data entry occurs.