Universe DEsigner is not getting correct Cardinality

I have two tables which are in a One-to-One relationship. But when I join them in Universe, the designer is saying invalid cardinality and detecting that join as one-to-many. I even created a primary key on both the tables to make sure it's one-to-one. Still it's throwing warning message as invalid Cardniality. Any ideas? btw, I am in BEXI 3.1