another sql problem

following is a sql statement i am trying to convert from oracle to informix. the sql statement that ran fine on in oracle, but it is giving me trouble on informix. from what i can tell from documentation and other posts to this newsgroup, the statement is written correctly. when i run the statment i get an error about trying to insert a null value into validrefills.last_rfdate. there are no nulls in the source table (ndc_data_in), so it seems that informix is trying to update the table with a non-existent record. it is like it isn't paying any attention to the where clause and trying to update every record in the table. do any of you informix gurus see any glaring mistakes in this sql? thanks, ty o'kelly tokelly@tcac.net update validrefills set (last_rfdate, rf_days, next_rf_date, calc_type, rx_expiration_date, rx_refill_left, cardholder_id, last_refill_nbr )= ((select max(rx_last_disp_date), max(rf_days), max(next_rf_date), max(calc_type), min(rx_expiration_date), min(rx_refill_left), max(rf_bm_cardhldr_id), max(rx_refill_nbr-rx_refill_left) from ndc_data_in n where n.ndc_store_nbr = validrefills.store_nbr and n.rx_number = validrefills.rxnum and n.new = 'N' )) where exists (select * from ndc_data_in n where n.ndc_store_nbr = validrefills.store_nbr and n.rx_number = validrefills.rxnum and n.new = 'N');