Insufficient Permissions! Subscribers aren't prompted to 'register'...??

When attempting to subscribe to our blog, users receive an error message saying "Insufficient Permssions! There was an error - you do not have the permission to subscribe to this blog" but there is no prompt for them to subscribe first - this is very unhelpful as its putting visitors off from subscribing on our site.

Is there some way to change this so that rather than an error message, they receive a more helpful "please register first in order to subscribe" message???

See http://www.phaseready.com/index.php/blog