SQL & PL/SQL

Why can't I grant execute to a directory?

I have carried out the following:

Login as SYS

Prove that the oracle directory exists SELECT * FROM all_directories WHERE directory_name = 'dir_name';

GRANT EXECUTE ON DIRECTORY dir_name TO role_name;

The following documentation at the following address says that I can grant execute:

http://www.morganslibrary.org/reference/directories.html

And yet when I directed the dba to do it and when I query the database to see if He did it it shows me that he did:

GRANTOR                        GRANTEE                        TABLE_SCHEMA                   TABLE_NAME                     PRIVILEGE                                GRANTABLE HIERARCHY 
------------------------------ ------------------------------ ------------------------------ ------------------------------ ---------------------------------------- --------- --------- 
SYSTEM                         role_name                    SYS                            dir_name                     EXECUTE                                  NO        NO        
SYSTEM                         role_name                    SYS                            dir_name                     READ                                     NO        NO        
SYSTEM                         role_name                    SYS                            dir_name                     WRITE                                    NO        NO        

3 rows selected

When I try it in my xe environment it throws the error:

Error starting at line 1 in command:
GRANT EXECUTE ON DIRECTORY dir_name TO role_name
Error report:
SQL Error: ORA-22928: invalid privilege on directories
22928. 00000 -  "invalid privilege on directories"
*Cause:    An attempt was made to grant or revoke an invalid privilege on a
           directory.
*Action:   Only CREATE, DELETE, READ and WRITE privileges can be granted or
           revoked on directories. Do not grant or revoke other privileges.

What is going on?

Benton