I get the error 'validation error for column DISCOUNTED, value
"***null***"At procedure ...'
The column definition is
DISCOUNTED D_BOOLEAN DEFAULT 'N' NOT NULL
So why wouldn't its value be set to N when I insert in to the table? Is
it because I'm explicitly trying to insert NULL for the column?

I don't know if this makes a difference, but the insert occurs in an
after trigger for another table.
If I add
IF (DISCOUNTED IS NULL) THEN DISCOUNTED = 'N';
Just before the UPDATE OR INSERT then it works.