There are two posibilities.
1. your column is set as NOT ALLOW NULL
2. your field value is null and your are consider that column in INSERT QUERY
Make your column as ALLOW NULL and check field value for NULL before insertion.
if field value is NULL, Do not consider that column in INSET query. Database will automatically put NULL in that column.
you can ALLOW NULL your column by
ALTER TABLE TABLENAME ADD COLUMNNAME text NULL
10 tricks for handling null values in microsoft access[
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