can't save legal information in postgresql

I use drupal 6.2 together with postgres 8.2.6. when i try to save my legal-information on /admin/settings/legal, i get this error-message:

* warning: pg_query(): Query failed: FEHLER: NULL-Wert in Spalte »tc_id« verletzt Not-Null-Constraint in /srv/www/www.example.de/includes/database.pgsql.inc on line 138.

* user warning: query: INSERT INTO legal_conditions (tc_id, conditions, date, extras, changes) VALUES (NULL, 'das ist ein test', 1213021206, 'a:5:{s:8:"extras-1";s:0:"";s:8:"extras-2";s:0:"";s:8:"extras-3";s:0:"";s:8:"extras-4";s:0:"";s:8:"extras-5";s:0:"";}', '') in /srv/www/www.example.de/sites/all/modules/legal/legal.module on line 316.