Bizzare plpgsql error

I'm getting this error trying to run a function:

usps=# select process_zip4();
WARNING: Error occurred while executing PL/pgSQL function process_zip4
WARNING: line 55 at SQL statement
ERROR: parser: parse error at or near "$1" at character 14

The relivant code is:

IF z.record_type_code = ''P'' THEN
INSERT INTO po_box
SELECT z4.zip4_id
, ....

The INSERT INTO is line 55. What makes no sense is that it works fine if
I change the name of the table from po_box to anything else.

Any ideas?
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Jim C. Nasby (aka Decibel!) jim(at)nasby(dot)net
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