FK pointing to an UNIQUE field. Problems to insert registers

I need insert a new record and the table where insert have 2 FK not null:
If I relate TABLE_A (FK) with TABLE_B (PK) Cayenne works well but if the relationship is between TABLE_A (FK) with TABLE_C (An UNIQUE field), cayenne can´t extract de value of that UNIQUE value, and the insert sentence generated live that field null and I get next error:

java.sql.SQLIntegrityConstraintViolationException: ORA-01400. Can not perform an insert NULL