Password Verify Function Oracle 10g

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Oracle

Well I created this function to verify the password

<pre> CREATE OR REPLACE FUNCTION dd_pwd_fun(username varchar2, password varchar2)
   --Do stuff
   END dd_pwd_fun;

And I created it using the sysadmin account, when I'm trying to alter the profile and assign it this function, I'm having an error:

ALTER PROFILE prof_name LIMIT
    PASSWORD_VERIFY_FUNCTION dd_pwd_fun;
    ERROR at line 1: 
    ORA-28004: invalid argument for function specified in PASSWORD_VERIFY_FUNCTION DD_PWD_FUN 

Why am I getting this? How can I add the function to the profile?