i was trying to update my moodle 1.9.9 (Build: 20100609) to 2.0 RC1. When updating the database following error occurs:

Upgrading to new version
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System
Increasing size of user idnumber field, this may take a while...
Migrated course files - course 1082/1082.

0 sec
100%
Migrated blog attachments - 202/209.

0 sec
96.65%
Fixing mnet records, this may take a while...
This upgrade introduces two new navigation blocks that will replace these blocks: Administration, Courses, Activities and Participants. If you had set any special permissions on those blocks you should check to make sure everything is behaving as you want it.

Error writing to database

More information about this error

Debug info: Data too long for column 'description' at row 1
UPDATE mdl_user SET description = ?,descriptionformat = ? WHERE id=?
[array (
0 => 'Mimi<br />
<br />
<br />
<br />
<br />
.
.
manny equal lines
.
.
<br />
<br />
<br /> ',
1 => '1',
2 => '7459',
)]
Stack trace:

• line 394 of /lib/dml/moodle_database.php: dml_write_exception thrown
• line 922 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
• line 954 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->update_record_raw()
• line 2403 of /lib/db/upgrade.php: call to mysqli_native_moodle_database->update_record()
• line 1307 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 253 of /admin/index.php: call to upgrade_core()

the table mdl_user has 15831 lines.