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!



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