Cached PL/PGSQL query plan failure

PostgreSQL 7.4.2 on i386-redhat-linux-gnu, compiled by GCC 2.96
OS: Redhat 7.2

Snippet of code which fails:

squery := ''SELECT '' || scolname || '' AS s_data FROM '' || stblname || ''
WHERE id = '' || idrow || '' LIMIT 1'';
FOR data_rec IN EXECUTE squery LOOP
IF (text(sdata) = text(data_rec.s_data)) OR (data_rec.s_data IS NULL AND
sdata IS NULL) THEN
RETURN VOID;
END IF;
END LOOP;

Error message:
ERROR: type of "data_rec.s_data" does not match that when preparing the plan

I get this message if I call the above code once per transaction and the
difference between the transactions is that the data type returned in the
dynamic query is not the same.
I would understand it better if I were calling this piece of code multiple
times with in the one transaction - but I am not.
To over come this problem I had to change the dynamic query as such:

squery := ''SELECT text('' || scolname || '') AS s_data FROM '' || stblname ||
'' WHERE id = '' || idrow || '' LIMIT 1'';
FOR data_rec IN EXECUTE squery LOOP
IF (text(sdata) = data_rec.s_data) OR (data_rec.s_data IS NULL AND sdata IS
NULL) THEN
RETURN VOID;
END IF;
END LOOP;

Regards
Donald Fraser