Error calling function which returns a ROWTYPE from within another function

I am experiencing unexpected behaviour on Postgres 9.0.4 using pl/pgsql relating to selecting from a function that returns a ROWTYPE into a ROWTYPE variable from within another function.
In the example below I :

1) Create a table, TESTTABLE and insert a row.
2) Create a function FN_TEST_GET_ROW that returns a row of ROWTYPE TESTTABLE based on selection of a single row from TESTTABLE
3) Create a test harness in the form of a function TESTX that calls FN_TEST_GET_ROW with ID=1
4) Call the test harness

The error shown below is returned unexpectedly
ERROR:  invalid input syntax for integer: "(1,Fred)"

I would just expect the values (1, Fred) to be returned which is what happens if I execute

SELECT fn_test_get_row(1);

directly.

See attached SQL file for all scripts.

Create table
============
CREATE TABLE testtable
(
id INTEGER,
name VARCHAR(10)
);

Add Data
========
INSERT INTO testtable (id, name) VALUES (1, 'Fred');

END;
$$ LANGUAGE plpgsql;

Create function
===============
CREATE OR REPLACE FUNCTION fn_test_get_row(a INTEGER)
RETURNS testtable AS $$
DECLARE
i_row testtable;
BEGIN

SELECT *
INTO   i_row
FROM testtable
WHERE id = a;

-- Success
RETURN i_row;

END;
$$ LANGUAGE plpgsql;

Create test function
====================
CREATE OR REPLACE FUNCTION testx()
RETURNS testtable AS $$
DECLARE
i_row testtable;
BEGIN

SELECT fn_test_get_row(1)
INTO   i_row;

-- Success
RETURN i_row;

Execute the test function
=========================
select testx();

Error returned
==============
ERROR:  invalid input syntax for integer: "(1,Fred)"
CONTEXT:  PL/pgSQL function "testx" line 8 at SQL statement

********** Error **********

ERROR: invalid input syntax for integer: "(1,Fred)"
SQL state: 22P02
Context: PL/pgSQL function "testx" line 8 at SQL statement