Problems creating indexes with IMMUTABLE functions

All,
It appears that the use of certain kinds of functions on columns to
create indexes is disabled in PG 7.4.1 (on RH 8.0 3.2-7). An attempt to
create an index on the concatenation of a date and an int gives:

sysd=> create index bnoz_idx on evidence (bnoz(alert_date, period));
ERROR: functions in index expression must be marked IMMUTABLE

although:

sysd=> select provolatile, prosrc from pg_proc where proname='bnoz';
provolatile | prosrc
-------------+------------------------------------------------------------------------------------------------------------------------------------------------
i |
declare
hr alias for $2;
pad text := '0';
begin
if hr<10 then
return $1 || ' ' || pad || hr;
end if;
return $1 || ' ' || hr;
end;

That is, the function is IMMUTABLE according to the catalog. The problem
appears to be with the date field:

sysd=> create index bnoz_idx on evidence ((alert_date || ' ' || period));
ERROR: functions in index expression must be marked IMMUTABLE

though:

sysd=> create index bnoz_idx on evidence ((service || ' ' || period));
CREATE INDEX

works, where 'service' is a text column.

I've searched the lists for indciations of this problem, and have failed
to find anything. Does this ring a bell with anyone?

Sincerely,
Rich Cullingford
rculling(at)sysd(dot)com