Error migrating database from 2.7.2 to 2.9 - python-2.7

Question

During migration of a CKAN instance from version 2.7.2 to 2.9 I'm facing the following error:
2022-03-11 14:11:28,312 INFO [ckan.cli] Using configuration file /etc/ckan/____/production.ini
2022-03-11 14:11:28,312 INFO [ckan.config.environment] Loading static files from public
2022-03-11 14:11:28,364 INFO [ckan.config.environment] Loading templates from /usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/templates
2022-03-11 14:11:28,581 INFO [ckan.config.environment] Loading templates from /usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/templates
Traceback (most recent call last):
File "/usr/lib/ckan/____/bin/ckan", line 11, in <module>
load_entry_point('ckan==2.9.5', 'console_scripts', 'ckan')()
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 829, in __call__
return self.main(*args, **kwargs)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 782, in main
rv = self.invoke(ctx)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 1259, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 1259, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/click/core.py", line 610, in invoke
return callback(*args, **kwargs)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/cli/db.py", line 64, in upgrade
_run_migrations(plugin, version)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/cli/db.py", line 122, in _run_migrations
repo.upgrade_db(version)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/model/__init__.py", line 326, in upgrade_db
alembic_upgrade(self.alembic_config, version)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/command.py", line 254, in upgrade
script.run_env()
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/script/base.py", line 427, in run_env
util.load_python_file(self.dir, 'env.py')
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
module = load_module_py(module_id, path)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/util/compat.py", line 135, in load_module_py
mod = imp.load_source(module_id, path, fp)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/migration/env.py", line 80, in <module>
run_migrations_online()
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/migration/env.py", line 74, in run_migrations_online
context.run_migrations()
File "<string>", line 8, in run_migrations
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
self.get_context().run_migrations(**kw)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
step.migration_fn(**kw)
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/migration/versions/001_103676e0a497_create_existing_tables.py", line 20, in upgrade
if skip_based_on_legacy_engine_version(op, __name__):
File "/usr/lib/ckan/____/local/lib/python2.7/site-packages/ckan-2.9.5-py2.7.egg/ckan/migration/__init__.py", line 22, in skip_based_on_legacy_engine_version
return int(version) >= int(filename.split(u'_', 1)[0])
ValueError: invalid literal for int() with base 10: 'None'
The skip_based_on_legacy_engine_version function is in this file from codebase. In this comparison int(version) >= int(filename.split(u'_', 1)[0]), the second returns 001, and the filename is the first of the versions folder.
I didn't get if the sqlalchemy migrate_version shoud maintain data stored in the Ckan database, couldn't find out.
The version should be setted on the alembic.ini file ?