problem when empty string is being passed to update_attributes

Hi,

I have the following problem:
I have a page where users can update their passwords.
When users just leave the password and password_confirmation fields
blank, rails will not raise an error when I do

@user.update_attributes(params[:user])

Instead, rails will just leave the record as it is.
My guess is that empty fields in the params[:user] hash are just being
ingored when calling update_attributes.
Any idea how to get around that?
I'm sure it's really easy but I have no idea how to fix that

thanks,
Marc

thanks