Package fails, execute in sql*plus works

Question

SQL> execute toa.toa_security.change_password('testuser','testpassword');
ALTER USER testuser IDENTIFIED BY testpassword
BEGIN toa.toa_security.change_password('testuser','testpassword'); END;
ERROR at line 1:
ORA-01031: insufficient privileges
ORA-06512: at "TOA.TOA_SECURITY", line 77
ORA-06512: at line 1
Here is the code
PROCEDURE CHANGE_PASSWORD (userid VARCHAR2, passwd VARCHAR2) IS
  sqlstr VARCHAR2(500);
BEGIN
  sqlstr :=
    'ALTER USER '||userid||' IDENTIFIED BY '||passwd;
  EXECUTE IMMEDIATE sqlstr;
END CHANGE_PASSWORD;Line 77: EXECUTE IMMEDIATE sqlstr;
No matter what user I run this as it fails (even the owner who has DBA priv). I have other procedures and functions in this package and they all run fine for the owner and a "simple" user. So obvioulsy it is not a privilege missing for the user on the package even though I did "grant execute on toa_security to <user>;".
I'm stumpped and baffled. And, I'm probably blind as to what is wrong.
Any ideas?

Answer 1

What William Robertson said is correct. Here's what happened on my system, I had to grant ALTER USER to myself directly:
SQL> show user
USER is "SYSTEM"
SQL> CREATE OR REPLACE
  2  PROCEDURE CHANGE_PASSWORD (userid VARCHAR2, passwd VARCHAR2) IS
  3
  4  BEGIN
  5
  6    EXECUTE IMMEDIATE 'ALTER USER '||userid||' IDENTIFIED BY '||passwd;
  7
  8  END CHANGE_PASSWORD;
  9  /
Procedure created.
SQL> exec change_password('HR','blah');
BEGIN change_password('HR','blah'); END;
ERROR at line 1:
ORA-01031: insufficient privileges
ORA-06512: at "SYSTEM.CHANGE_PASSWORD", line 5
ORA-06512: at line 1
SQL> grant alter user to system;
Grant succeeded.
SQL> exec change_password('HR','blah');
PL/SQL procedure successfully completed.
SQL> conn hr/blah
Connected.
SQL>