WARNING: group with ID NNN does not exist

I'm getting the following warning on 8.0beta3:

WARNING: group with ID 103 does not exist

It is true that I don't have a pg_group.grosysid = 103,
but I don't understand why the backend expects it in the
first place. Here's an example:

% psql -c "create user \"root(at)foobar\""
CREATE USER
% psql -c "select usename, usesysid from pg_user where usename = 'root(at)foobar'"
usename | usesysid
-------------+----------
root(at)foobar | 268
(1 row)

% psql -c "alter group perfmon_agent_group add user \"root(at)foobar\""
ALTER GROUP

% psql -c "select * from pg_group where groname = 'perfmon_agent_group'"
groname | grosysid |
grolist

---------------------+----------+---------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------------------------
------------------------
perfmon_agent_group | 101 | {196,265,266,197,198,199,200,201,202,203,204,205,206,207,208,267,209,210,211,212,264,213,214,215,216,217,218,219,220
,221,222,223,224,225,226,195,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,249,250,251,252,253,254,255,256,257,2
58,259,260,261,248,268}
(1 row)

% psql -U "root(at)foobar" -c "insert into measurement_error(host_id, measure_id, error, source_id, local_source_id) values (37, 23, 'foo', 1, 1)"
WARNING: group with ID 103 does not exist
INSERT 111854687 1

Is "root(at)foobar" and illegal username? Other ideas?

TIA.

Ed