The code in parse-url for internet-url that which picks apart the domain name and optional port number, user name, and password dies in a state of confusion due to the colon in this example.

(araneida:parse-urlstring "http://a.us/:")

taking a blind stab at the problem, changing the binding of colon1-s to this:

(colon1-s (position #: string :end (or at-s dir-s)))

hides, and possible fixes, the error.