Nested repeater fields?

I have the pro version and so far so good except I have a repeater field with three sub-fields, one of which is another repeater. I can add the two non-repeater sub-fields, but the repeater isn’t showing to be able to be added. And of course if I can’t add that, then I can’t add its subfields. Is there something I’ve missed or are repeaters as sub-fields not supported?

Cheers