Not a valid month error

Seems that the error occurs within the function. We are hitting some strange error on date field which causes all the existing view not working. All these views rely on a function like below. But it keeps giving "Not a valid month" error and return nothing. Our database NLS date format is DD-MON-RR in traditional chinese. FUNCTION test ( n_mortgage_id IN MORTGAGE.mortgage_id%TYPE, dt_effective_date IN MORTGAGE_TERM.effect_date_from%TYPE ) RETURN DATE AS c_term_date SYS_REFCURSOR; dt_term_date MORTGAGE_TERM.effect_date_from%TYPE; BEGIN IF dt_effective_date = '01-JAN-00' THEN