ERROR: cache lookup for type 0 of ... pg.dropped.14........ failed

Hi All,
I run into an error message.
I have the following function where t_stockchanges is one of my tables.
--------------------------------------------
create or replace function get_last_open(int, int) returns setof
t_stockchanges AS '
declare
ProductID alias for $1;
StockID alias for $2;

q text;
Rec t_stockchanges%ROWTYPE;
begin
q := ''select * from t_stockchanges where '' ||
''stockid='' || quote_literal(StockID) || '' and '' ||
''productid='' || quote_literal(ProductID) || '' and '' ||
''changeid=1 order by time desc limit 1;'';
for Rec in execute q loop
return next Rec;
end loop;
return;
end;'
language 'plpgsql';
--------------------------------------------

Running the following query I get the error message:
#> select * from get_last_open(140,1);

WARNING: plpgsql: ERROR during compile of get_last_open near line 5

ERROR: cache lookup for type 0 of
t_stockchanges.........pg.dropped.14........ failed

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Could anybody tell me how can I work it around. A tried to restart the
server to make empty the cache, but it seems to be worthless.

Thank you in advance.

Bye,
-- Csaba Együd