Array push with associate array

If I am working with an associate array like such:

Array ( [Username] => user 
        [Email] => email 
      )

and I want to add an element to the end, I would think to do:

array_push($array, array('Password' => 'pass'));

However, this leaves me with:

Array ( [Username] => user 
        [Email] => email
        Array ( [Password] => pass )
      )

How can this be avoided so I end up with:

Array ( [Username] => user 
        [Email] => email
        [Password] => pass
      )

Much appreciated!