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(at)pcorp(dot)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.