ERROR: fmgr_info: function 24809: cache lookup failed

Hi all. I feel awfully stupid.

Migrating my skillset from Oracle to Postgres, I am trying (in vain) to
create a trigger and a sequence to "auto increment" the primary key in a
table, so I won't have to worry about setting it; each time you INSERT
into the table, the primary key will be automagically assigned.

Problem is, it isn't working... I get "cache lookup failed" whenever I try
to INSERT on that table!

THE TABLE:
CREATE TABLE SENTENCES (
SENTENCEID NUMERIC,
CHAPTERID NUMERIC,
SOFFSET INTEGER,
PRIMARY KEY (SENTENCEID)
);

THE SEQUENCE:
CREATE SEQUENCE sentenceid_seq;

THE FUNCTION:
CREATE FUNCTION sentenceid_fcn()
RETURNS OPAQUE AS '
BEGIN
SELECT nextval(sentenceid_seq)
INTO :new.sentenceid
FROM dual;
END; ' language plpgsql;

THE TRIGGER:
CREATE TRIGGER sentenceid_trig
BEFORE INSERT
ON sentences
FOR EACH ROW
EXECUTE PROCEDURE sentenceid_fcn();
END;

And when I try to INSERT into SENTENCES, without my SENTENCEID, hoping
that the function/trigger/sequence will kick in and assign it for me...

architxts=# INSERT INTO SENTENCES (CHAPTERID, SOFFSET) VALUES (123, 147);
ERROR: fmgr_info: function 24809: cache lookup failed

I'm certain I'm doing SOMETHING terribly wrong. Could someone help?
PLEEEEASE? Why am I getting that error?

Many thanks in advance!

--
J e s s i c a L e a h B l a n k