argument of AND must not return a set when using regexp_matches

I've been getting a funny SQL error, which I've boiled down to this case.

SELECT (regexp_matches('abc', '(.)b(.)'))[1] IS NOT NULL
-- Returns true, as expected

SELECT (regexp_matches('abc', '(.)b(.)'))[1] IS NOT NULL AND true
-- Gives this error:
ERROR: argument of AND must not return a set
SQL state: 42804

Can anyone make heads or tails of it? Is it a real bug? Is there a work around?

Postgres 8.3