Why ths works in anonymous but not in a procedure?

If I do the following, everything is fine:

declare
  l_foo clob;
begin
select
regexp_replace(
  dbms_metadata.get_ddl('USER', 'SCOTT', null) ||
  dbms_metadata.GET_GRANTED_DDL ('SYSTEM_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('OBJECT_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('ROLE_GRANT', 'SCOTT')
,'"' || chr(10), '";' || chr(10)) 
into l_foo 
from dual;
end;
/

But if I wrap this in a procedure:

create procedure tests is
  l_foo clob;
begin
select
regexp_replace(
  dbms_metadata.get_ddl('USER', 'SCOTT', null) ||
  dbms_metadata.GET_GRANTED_DDL ('SYSTEM_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('OBJECT_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('ROLE_GRANT', 'SCOTT')
,'"' || chr(10), '";' || chr(10)) 
into l_foo 
from dual;
end;
/

And execute the procedure by "exec tests;" then I catch a object SCOTT of type USER not found in schema SCOTT.

Why is this and how do I get around?

Thanks Chris

Oracle Documentation states:

In stored procedures, functions, and definers-rights packages, roles (such as SELECT_CATALOG_ROLE) are disabled. Therefore, such a PL/SQL program can only fetch metadata for objects in its own schema. If you want to write a PL/SQL program that fetches metadata for objects in a different schema (based on the invoker's possession of SELECT_CATALOG_ROLE), you must make the program invokers-rights.

To do this, you must add authid to your procedure.

create procedure tests authid CURRENT_USER is
  l_foo clob;
begin
select
regexp_replace(
  dbms_metadata.get_ddl('USER', 'SCOTT', null) ||
  dbms_metadata.GET_GRANTED_DDL ('SYSTEM_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('OBJECT_GRANT', 'SCOTT') ||
  dbms_metadata.GET_GRANTED_DDL ('ROLE_GRANT', 'SCOTT')
,'"' || chr(10), '";' || chr(10)) 
into l_foo 
from dual;
end;
/

SQL> EXEC tests 

PL/SQL procedure successfully completed