TNS:could not resolve the connect identifier specified

We are using one DB link to connected to another database, when we try to run below query from pl/SQL developer in one window I am getting output and in another window I am betting error " "

select * from dual@db_link;

in one window <----- output is X

in another window  <------------ TNS:could not resolve the connect identifier specified

Kindly help us in solving out.

Regards,

Surya Prakash.