Setting DATE Field To NULL

I am loading data from a delimited ASCII file to Informix via the 4GL load command. The ASCII delimited file is created by COBOL, so I do have control over how the DATE fields are written. I need to be able to insert NULLs into DATE fields for rows with invalid data. If I try to set the date field to spaces, I get an error about the month not being valid. Is there a particular value to set a date field to in an ASCII file so 4GL or DBLoad will interpret the column contents as NULL? Thanks. Mitch