Nested xpath() calls "lose" namespace support

I was trying to extract XML fragments within a pl/pgsql function by nested
xpath() function calls, and found that when namespaces are used in the
XML, only the first xpath() call is able to correctly use namespaces.

First here is an example that works, when no namespaces are used:

BEGIN;
CREATE TEMPORARY TABLE tmp_xpath_test (x xml) ON COMMIT DROP;
INSERT INTO tmp_xpath_test VALUES (
    '<foo><bar x="y">bar1</bar><bar x="y">bar2</bar></foo>'::xml);
SELECT (xpath('/foo/bar[1]', t.x))[1] FROM tmp_xpath_test t;
SELECT xpath('/bar/@x', (xpath('/foo/bar[1]', t.x))[1]) FROM
tmp_xpath_test t;
COMMIT;

The first select is just there to show the result of the inner call to
xpath() in the second select, and the second select returns:

SELECT xpath('/bar/@x', (xpath('/foo/bar[1]', t.x))[1]) FROM
tmp_xpath_test t;
 xpath
-------
 {y}
(1 row)

Now if I use XML with namespaces, the first SELECT works, but the second
never returns the expected results:

BEGIN;
CREATE TEMPORARY TABLE tmp_xpath_test (x xml) ON COMMIT DROP;
INSERT INTO tmp_xpath_test VALUES (
    '<a:foo xmlns:a="a:urn"><a:bar x="y">bar1</a:bar><a:bar
x="y">bar2</a:bar></a:foo>'::xml);
SELECT (xpath('/a:foo/a:bar[1]', t.x, ARRAY[ARRAY['a','a:urn']]))[1] FROM
tmp_xpath_test t;
SELECT xpath('/a:bar/@x', (xpath('/a:foo/a:bar[1]', t.x,
ARRAY[ARRAY['a','a:urn']]))[1],
    ARRAY[ARRAY['a','a:urn']]) FROM tmp_xpath_test t;
COMMIT;

The select results are

SELECT (xpath('/a:foo/a:bar[1]', t.x, ARRAY[ARRAY['a','a:urn']]))[1] FROM
tmp_xpath_test t;
           xpath
---------------------------
 <a:bar x="y">bar1</a:bar>
(1 row)

SELECT xpath('/a:bar/@x', (xpath('/a:foo/a:bar[1]', t.x,
ARRAY[ARRAY['a','a:urn']]))[1],
lms_kia(> ARRAY[ARRAY['a','a:urn']]) FROM tmp_xpath_test t;
 xpath
-------
 {}
(1 row)

For the second select, I expected a single XML text node containing "y",
just like from the no-namespace result.

Am I doing anything wrong here?

-- m@