11gr2: grant create session results in invalid username.

Hello,

I created a new user in oracle database,     I am trying to grant privilege , but it results in  invalid username

select username,profile,account_status from dba_users where username = 'XXXXX';

XXXXX USER_PROFILE OPEN

SQL> grant create session to 'XXXXX';

grant create session to 'XXXXX'

                        *

ERROR at line 1:

ORA-00987: missing or invalid username(s)

Please advise

Thanks

Ram