Switch to another user within sqlplus

Hello,

Oracle 11.2

How can I switch to another user in sqlplus when I connected using Os authentication as Oracle.

$> sqlplus / as sysdba

SQL> connect CUSTOMER;

Enter password:

ERROR:

ORA-01005: null password given; logon denied

When it prompted for password I just pressed enter. Should I specify sys password or maybe there is other way to switch ?

I do not know password for sys and CUSTOMER user.

Thanks in advance,