Interval - incorrect grammar form

Hi,
I've noticed that interval is shown incorrectly (in plural form) when date
is negative:

select age(timestamp '2015-09-01',timestamp '2017-10-02');
gives:
"-2 years -1 *mons* -1 days"

This mistake isn't present when date is positive
select age(now(),'2015-06-07');

"1 *mon* 9 days 11:53:37.567851"

It applies to versions:
"PostgreSQL 9.4.1 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2
20080704 (Red Hat 4.1.2-55), 64-bit"
and
"PostgreSQL 8.4.17 on i486-pc-linux-gnu, compiled by GCC gcc-4.4.real
(Debian 4.4.5-8) 4.4.5, 32-bit"

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