SQL Question

In 7.0.3 I want to do:

select count( distinct browser, ssn) from applicant_sessions where browser
like '%5.5%';

But I get this error:

ERROR:  Aggregate functions may only have one parameter

In 7.1 I can do:
select count(*) from (select distinct browser, ssn from applicant_sessions
where browser like '%5.5%') as distinctbrow;

to get the desired result, but I can't do that in 7.0.x.  Any suggestions?
Am I doing anything wrong with my sql syntax.  I'm working around it right
now by doing a pg_numrows from PHP after I do the first select, but I would
like to get the count directly as it will be faster.

Thanks,

Matt