Tosefta к Кидушин 4:16

(These are those who are doubtful: shetuki, asufi and kuti.)(—missing in Ehrfurt manuscript, supplied from Mishnah Kiddushin 4:3). And so also Rabbi Elazar (sic! based on Ehrfurt) used to say: A mamzer can't marry a kutit, a kuti can't marry a mamzeret, and a kuti can't marry a kuti—so too with shetuki, asufi etc.