upgrade of D6.17 to D7.0alpha6 failed using postgreSQL

I have a working d6.17 site using PostgreSQL 8.3.11
As an upgrade test I duplicated my site (database, web setup) and followed instructions from UPGRADE.txt of latest alpha release.

When I opened /update.php I faced the following error message:

Error message
PDOException: SQLSTATE[42601]: Syntax error: 7 ERROR: syntax error at or near "schema_version" LINE 1: ALTER TABLE system ALTER schema_version SET schema_version =... ^: ALTER TABLE {system} ALTER schema_version SET schema_version = CAST(schema_version_old as smallint); Array ( ) in db_change_field() (line 2841 of /includes/database/database.inc).
The website encountered an unexpected error. Please try again later.

I tried to figure out what could be the issue here so checked system table. Please see attached query output.

If you need more information to fix this issue contact me.