SQL & PL/SQL :: Call To Function In SYS Schema

I am defining a function in a schema as a user with admin role (default). The function is to do a call to another function on a custom package in SYS schema.

something funny happens : when I compile the function (defined with authid as DEFINER)it says that the function SYS.custom_package.myFunction is not defined.

however if I do a select sys.custom_package.my Function from dual it's okay.

why this behaviour and how to work around it? You see, the package in SYS proposes a number of other functions that I don't want to expose. I only intend to create some sort of wrapper function that would Marshall the call to the my Function only on the custom_package in SYS.

my wrapper function looks something like that :

create or replace function myFunction_wrapper authid definer return pls_integer is
begin
return sys.custom_package.myFunction;
end;