Failed assertion on cluster

I get the following:

$ TRAP: FailedAssertion("!(!(tup->t_data->t_infomask & 0x0010))", File: "heapam.c", Line: 1133)


when I try to cluster this table:

CREATE TABLE virtusers (    lhs text,    rhs text,    insert_date timestamp(0) with time zone DEFAULT now(),
insert_whotext DEFAULT "current_user"(),    "comment" text
 
);
ALTER TABLE ONLY virtusers ALTER COLUMN lhs SET STATISTICS 100;

--
-- Name: vu_lhs_index; Type: INDEX; Schema: public; Owner: ler; Tablespace: 
--

CREATE UNIQUE INDEX vu_lhs_index ON virtusers USING btree (lhs);


ALTER INDEX public.vu_lhs_index OWNER TO ler;


When I issue the cluster vu_lhs_index on virtusers, I get the 
above assertion.

8.0.1 on UnixWare 7.1.4


-- 
Larry Rosenman                     http://www.lerctr.org/~ler
Phone: +1 972-414-9812                 E-Mail: ler@lerctr.org
US Mail: 1905 Steamboat Springs Drive, Garland, TX 75044-6749