Error Occurred when run function. How to solve it?

Error Occurred when run function.  Please give me some advices.  Thanks.

The function:
-- Function: usf_annatest(varchar, varchar)
CREATE FUNCTION usf_annatest(varchar, varchar) RETURNS varchar AS '
DECLARE
  curs_userdetail refcursor;
  table_name ALIAS for $1;
  identity ALIAS for $2;
  query_where VARCHAR;

BEGIN
  IF table_name = "ot_customer" THEN
      RETURN table_name;
  ELSE
      RETURN identity;
  END IF;
END;'  LANGUAGE 'plpgsql';

I run it :
SELECT usf_annatest('ot_test', 'anna');

Error :
NOTICE: Error occurred while executing PL/pgSQL function usf_annatest
NOTICE: line 8 at if
ERROR : Attribute 'ot_customer' not found.

So, when I need to user "ot_customer" and 'ot_customer'.

Thanks a lot.