SQL & PL/SQL

Hi,

I recently asked on here about creating my first stored procedure, it compiled fine, but unfortunately whilst trying to run it I get this error:

Retrieve Error:ORA-00904: "V_URN": invalid identifier
ORA-06512: at "CONTACT_RETURN", line 23
ORA-06512: at line 1

Here's my stored procedure:

CREATE OR REPLACE
procedure contact_return(
    v_urn IN number,
    p_cursor OUT SYS_REFCURSOR )
AS
  sql_statement VARCHAR2(4000) := '       

  SELECT URN,
    FIRSTNAME,
    LASTNAME,
    TITLE,
    CREATED_DT,
    AREA_URN,
    MOBILE,
    WORK,
    EMAIL,
    ORG_NAME,
    ADDRESS,
    POSTCODE,
    IN_USE
  FROM CONTACT
  WHERE urn = NVL(:1,v_urn)';
BEGIN
  OPEN p_cursor FOR sql_statement USING v_urn;
end;

I cannot see a problem with it, since v_urn is declared at the top in the brackets.

Does anyone please have any idea why I'm getting this error? I'm at a loss!

Thanks