OpenERP - Timesheet module - Hours encoding error

When trying to encode new hours in one's timesheet in OpenERP, the following error might appear:
ERROR:  null value in column "journal_id" violates not-null constraint insert into "account_analytic_line" (id,perm_id,"user_id","account_id","general_account_id","product_uom_id","journal_id","name","to_invoice","amount","unit_amount","date","product_id",create_uid,create_date) values (4,NULL,'4','6','634','4',NULL,'Analysis of AIM development project',NULL,-40.000000,2.000000,'2008-08-14','1',4,now())
The error code is not clear, but the problem in this case comes from t