CVS head currently has a single upper-case NIL in swank-rpc.lisp that prevents it from working with Allegro CL's mlisp. Downcasing that single token fixes the breakage.
It's at line 103 of last night's swank-rc.lisp:
(defparameter *validate-input* NIL "Set to true to require input that strictly conforms to the protocol")
Changing this form thusly makes mlisp happy again:
(defparameter *validate-input* nil "Set to true to require input that strictly conforms to the protocol")