strange error when inserting via a SRF into a table with a foreign key constraint

I got the error mesage,

ERROR:  could not find relation 19693 among query result relations,

from a strange interaction between inser, foreign keys and a udf.
After a but of trial and error, I came up with a test case:

create or replace function explode_array(in_array anyarray) returns
setof anyelement as
$$
    select ($1)[s] from generate_series(1,array_upper($1, 1)) as s;
$$
language sql immutable;

create table fp1(a text, b text, primary key(a,b));
create table foo (a text, b numeric, c text);

insert into foo select 'a', 1, explode_array(string_to_array('a,b',
',')); -- works
truncate foo;

alter table foo add foreign key(a,b) references fp1(a,b) on delete cascade;

insert into foo select 'a', 1, explode_array(string_to_array('a,b',
',')); -- fails

I confirmed this on 8.1.4.

merlin