fmgr_info: function 7390843: cache lookup failed

I am getting this error:

fmgr_info: function 7390843: cache lookup failed

when trying to insert some data into a table using the tsearch2 contrib
module (this is postgresql 7.3). I have the following trigger defined:

BEGIN
NEW.search_vector :=
setweight(to_tsvector(coalesce(NEW.display_text, '')), 'A') ||
to_tsvector(coalesce(NEW.content_desc, ''));
RETURN NEW;
END;

Which I suspect is causing the problem. Where can I look up that
function number to verify this? Any suggestions?

culley