error adding sales transaction.

["J Kinable" <..hidden..>]

Good morning,

I'm failing to add a new sales transaction and I'm not sure what's
going wrong. After going to sales->add transaction, fill in all the
fiels, click post and click post a again to confirm, I receive the
following error:
DBD::Pg::st execute failed: ERROR: null value in column "chart_id"
violates not-null constraint Error!
INSERT INTO acc_trans (trans_id, chart_id, amount, transdate) VALUES
(?, (SELECT id FROM chart WHERE accno = ?), ?, ?) ERROR: null value in
column "chart_id" violates not-null constraint

Screenshots of the problem:
http://nemesisclan.snt.utwente.nl/ledger_picture_2.png
http://nemesisclan.snt.utwente.nl/ledger_picture_3.png
Anyone any idea how I can solve this problem quickly? Please be
specific in your solution description since I'm not yet very familiar
with this program.

Thnx in advance,

Joris