ORA-12162 When Logging in Using BEQ

If you get ORA-12162 when you try to connect as sysdba:

When trying to connect locally (with BEQ or bequeath protocol) to the database shows "ORA-12162 error tns:net service name is incorrectly spelled"

This is unusual because a TNS:Net service name is not being specified, nor is the environement variable TWO_TASK set.

SQL*Plus: Release 11.1.0.6.0 - Production on Wed Aug 15 16:23:34 2007

Copyright (c) 1982, 2007, Oracle. All rights reserved.

SQL> conn sys/oracle as sysdba
ERROR:
ORA-12162: TNS:net service name is incorrectly specified


SQL> conn / as sysdba
ERROR:
ORA-12162: TNS:net service name is incorrectly specified

This occurs if environment varialble ORACLE_SID is not set. Once you set ORACLE_SID, the error goes away.