dynamic plpgsql question

Hi -

I am struggling with a trigger function in plpgsql, and am hoping that
someone on this list can't show me a way to do what I need.

In the trigger, TG_ARGV[0] is the name of a column that I want to
evaluate. This code shows the concept, though is not functional:

CREATE OR REPLACE FUNCTION foo() RETURNS TRIGGER AS $$
DECLARE
column_name TEXT := TG_ARGV[0];
data TEXT;
BEGIN
EXECUTE 'SELECT NEW.' || column_name INTO data;
-- ...
END;
$$ LANGUAGE plpgsql;

When I try to use that code, I receive:

c3i=> insert into test_table values (1,1);
ERROR: NEW used in query that is not in a rule
CONTEXT: SQL statement "SELECT NEW.magic"

How can I get the value of NEW.{column_name} (aka NEW.magic in this
specific test case) into the variable data?

For completeness, I am using version 8.2.0.

Thanks in advance - Marc