BUG #6053: Can't do DISTINCT on citext column

The following bug has been logged online:

Bug reference:      6053
Logged by:          Regina
Email address:      lr@pcorp.us
PostgreSQL version: 9.1beta1
Operating system:   Windows
Description:        Can't do DISTINCT on citext column
Details:

I have a table I created like this:
-- With and without the COLLATE in there I get the same error.

 CREATE TABLE test
(
  fullname citext COLLATE pg_catalog."POSIX",
);

INSERT INTO  test(fullname) VALUES('test1'),('test2');

When I do a:
SELECT DISTINCT fullname FROM test;

or a

SELECT DISTINCT fullname COLLATE pg_catalog."POSIX" FROM test;


It gives error:

ERROR:  could not determine which collation to use for lower() function
HINT:  Use the COLLATE clause to set the collation explicitly.