Please help! ORA-01031: insufficient privileges

In a PL/SQL procedure created under user ABC I do: execute immediate 'alter index ABC.MY_INDEX rebuild online'; And I get this:
ERROR at line 1:
ORA-01031: insufficient privileges
ORA-06512: at "ABC.MY_PROCEDURE", line 33

What is the problem? The table and the index are owned by ABC. I can do it at the SQL> prompt when connected as ABC or SYS. I know there is this problem about having privileges set by a role instead of a direct grant, but in this case the object belong to ABC and ABC executes the procedure. I have tried:
grant all on idx_rebuild_info to sys;
but that does not help either.

What am I doing wrong?

Thanks!

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