ERROR: unrecognized time zone name: "UTC"

I am experiencing this error trying to set the connection time zone to
UTC on PostgreSQL 8.4.4:

    postgres=# set time zone 'UTC';
    ERROR:  unrecognized time zone name: "UTC"

I have read the documentation for timezone_abbreviations, and
everything looked in order, at least as far as my limited knowledge
takes me:

    postgres=# show timezone_abbreviations;
     timezone_abbreviations
    ------------------------
     Default
    (1 row)


    $ grep UTC `pg_config --sharedir`/timezonesets/Default
    UTC         0    # Coordinated Universal Time

Setting the time zone to 'CST6CDT' appears to work fine:

    postgres=# set time zone 'CST6CDT';
    SET

Is there something I'm doing wrong here? I have a slightly odd
configuration (Gentoo Prefix Portage on Mac OS X, with a custom
ebuild) so the problem may well be with my system setup. But I'd
appreciate any pointers folks may have as to where to troubleshoot.

Thanks!
-be