cast lo to oid

Hi,

I have the following table

create table scan_docs (
docid serial,
shipno numeric(10),
scan lo,
type text
);

when I try to create the following rule

create rule "delete_scan_docs_lo" as
on delete to "scan_docs"
do select lo_unlink (old.scan);

I get the error message

"psql:scan_docs_rule.sql:3: ERROR: Function 'lo_unlink(lo)' does not exist
Unable to identify a function that satisfies the given argument types
You may need to add explicit typecasts"

So I add a type cast

create rule "delete_scan_docs_lo" as
on delete to "scan_docs"
do select lo_unlink (old.scan::oid);

but I get the following error message

"psql:scan_docs_rule.sql:3: ERROR: Cannot cast type 'lo' to 'oid' "

Is there away around this?

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