mysqladmin not taking inline password

Question

I am trying to set a cron job for taking backup from my slave machine. So I need to stop the slave

I issued a command

mysqladmin --user=root --password=test_pass stop-slave

But it is throwing error:

mysqladmin: connect to server at 'localhost' failed error: 'Access denied for user 'root'@'localhost' (using password: YES)'

Now I tried with the command

mysqladmin --user=root --password stop-slave

It prompts for password and I gave as test_pass and everything was fine.

Why so happened? What is the alternative?

NB: BTW my mysql version is mysql-5.0.95-5, it it make any sense.