root page 3 of index "pg_class_oid_index" has level 0, expected 1

Hi,

During major version upgrade from PostgreSQL v9.4 to v9.6 I facing
following error:

Performing Consistency Checks
-----------------------------
Checking cluster versions ok

connection to database failed: FATAL: root page 3 of index
"pg_class_oid_index" has level 0, expected 1

could not connect to old postmaster started with the command:
"/postgresql94/bin/pg_ctl" -w -l "pg_upgrade_server.log" -D
"/data/directory/postgresql" -o "-p 5432 -b -c listen_addresses='' -c
unix_socket_permissions=0700 -c
unix_socket_directories='/home/postgresql_user'" start
Failure, exiting

Again, I was trying to take the pg_dumpall of the same database
cluster(PostgreSQL v9.4) and it failed with error:
pg_dump: [archiver (db)] connection to database "template1" failed: FATAL:
root page 3 of index "pg_class_oid_index" has level 0, expected 1
pg_dumpall: pg_dump failed on database "template1", exiting

Please suggest how should I proceed with major version upgrade in this case.

Regards,
Abhijit