Error: timestamp with timezone + interval is not immutable while creating index

Hi All

I've got a table with (amongst others) two fields:
last_updated timestamp with time zone;
update_cycle interval;

I'd like to create an index on these, to index time "next update" time
(last_updated + update_cycle).

When I try this I get an error though:

main=> create index foo_next_update on foo( (last_updated + update_cycle) ) ;
ERROR:  functions in index expression must be marked IMMUTABLE

Does anyone know why adding two fields like this results in anything
other than an immutable function?  Under what circumstances could it
return a different result?

Thanks very much for any help.