Logical (boolean) field empty for no apparent reason

I ran a nightly pgdbf script each night since a while without any issue.  But, since one week, one logical field on a foxpro dbf appear with no value in the sql file which cause an error when I run that file in PSQL causing importation to abort.
When I look in this table in VFP, I see that this field is populate with a F.
Have you an idea what could cause that problem.
Thank
