Unexpected SQL error for UPDATE

Originally I have a table like this<br /><br /><br />create table users ( <br /> userid integer not null, <br />
email_addressvarchar (255) unique not null,<br /> PRIMARY KEY (userid) <br /> ); <br /><br /><br />Later I find that
varchar(255)is not enough. I designed to change the type to<br />text. There is no simple SQL to alter the type. So I
usea series of SQLs to<br />create a new column, copy the data over and then replace the old column with the <br
/>new.<br/><br /> ALTER TABLE users ADD email_address_text text UNIQUE; <br />UPDATE users set
email_address_text=email_address;<br/>ALTER TABLE users DROP email_address;<br />ALTER TABLE users RENAME
email_address_textTO email_address;<br />ALTER TABLE users ALTER email_address SET not null;<br /><br /><br />This
worksmostly OK. Until I have one database that has over 1 million records<br />in table user. It fails with an
inexplicableerror:<br /><br /><br />mydb=# UPDATE users set email_address_text=email_address;<br />ERROR: invalid page
headerin block 6776 of relation "users_email_address_text_key" <br /><br /><br />Anyone can shred some light what has
wentwrong?<br /><br />wy<br /><br /><br /><br />