phpBB 3.0.12 on Oracle fails to upgrade to 3.1.0

While attempting to upgrade a 3.0.12 board running on Oracle, the following error is encountered:

Updating database to latest stable release

Database type :: oracle
Previous version :: 3.0.12
Installed Data: \phpbb\db\migration\data\v310\style_update_p2; Time: 0.09 seconds

SQL ERROR [ oracle ]

ORA-01439: column to be modified must be empty to change datatype [1439]

SQL

ALTER TABLE phpbb_users MODIFY user_timezone varchar2(100) DEFAULT ''

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/phpbb/db/driver/driver.php
LINE: 748
CALL: trigger_error()

FILE: [ROOT]/phpbb/db/driver/oracle.php
LINE: 418
CALL: phpbb\db\driver\driver->sql_error()

FILE: [ROOT]/phpbb/db/tools.php
LINE: 1506
CALL: phpbb\db\driver\oracle->sql_query()

FILE: [ROOT]/phpbb/db/tools.php
LINE: 2586
CALL: phpbb\db\tools->_sql_run_sql()

FILE: [ROOT]/phpbb/db/tools.php
LINE: 812
CALL: phpbb\db\tools->sql_column_change()

FILE: (not given by php)
LINE: (not given by php)
CALL: phpbb\db\tools->perform_schema_changes()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 457
CALL: call_user_func_array()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 403
CALL: phpbb\db\migrator->run_step()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 242
CALL: phpbb\db\migrator->process_data_step()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 204
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/phpbb/db/migrator.php
LINE: 155
CALL: phpbb\db\migrator->try_apply()

FILE: [ROOT]/install/database_update.php
LINE: 234
CALL: phpbb\db\migrator->update()