Emptying a PL/SQL TABLE

In the PL/SQL User's Guide and Reference (Release 2.2), on p. 2-43, it says:

> However, you can use a simple workaround to delete an entire
> PL/SQL table. Simply assign NULL to it, as shown in the following
> example:

However, as the following transcript shows, you can't assign NULL to a PL/SQL table. I guess the manual is wrong :-(

Does anyone know if it is, in fact, possible to empty a PL/SQL table?

Connected to:
Oracle7 Server Release 7.2.3.0.0 - Production Release PL/SQL Release 2.2.3.0.0 - Production  

SQL> DECLARE   2 TYPE mytype IS TABLE OF NUMBER INDEX BY BINARY_INTEGER;   3 mytable mytype;
  4 BEGIN
  5 mytable := NULL;
  6 END;
  7 /
  mytable := NULL;

             *
ERROR at line 5:

ORA-06550: line 5, column 14:
PLS-00382: expression is of wrong type
ORA-06550: line 5, column 3: