Related for Bekhorot 60:2

An am ha'aretz who takes upon himself all the obligations of being a a chaver except for one of them, we do not accept him. A convert [others have Gentile (גוי)] who takes upon himself all the obligations of the Torah except for one of them, we do not accept him. Rabbi Yosei son of Rabbi Yehudah says, [that is true] even if it is a small thing from [among the] fine points in rabbinic enactments [following Jastrow re "דקדוקי סופרים"].