If you have more than one customer with a blank code, then you have a problem.
Open your pervasive control panel and paste this sql script:
select * from BKARCUST where ASCII(LTRIM(RTRIM(BKAR_CUSTCODE))) = 0
It should only show one single record (Your customer with blank code)
If there is only one, I will recommend you to duplicate the customer, this time with a proper customer code.
Then you can use another sql script to update your part table (MTICMSTR)
Kelloggs
PD: A field that identify a record should never be "blank"