help with nullif clause in ctl file

[ddiamond]

I have the following code:

EXPIRATION_DATE POSITION(205:212) DATE "YYYYMMDD" NULLIF EXPIRATION_DATE   = "        ",

I also want expiration_date to be null if the field bound<>'B', but I can't figure out how to include 2 nullif conditions. The following work-around seems to work, but I'd like to find something cleaner.

EXPIRATION_DATE POSITION(205:212) DATE "YYYYMMDD" NULLIF
EXPIRATION_DATE = "        " "DECODE(:BOUND,'B',:EXPIRATION_DATE)",