[plt-scheme] override error

"PLT MrED: Graphical Toolbox Manual" offers this code snippet:

(define append-only-text%
   (class text%
     (inherit last-position)
     (override can-insert? can-delete?)
     (define (can-insert? s l) (= s (last-position)))
     (define (can-delete? s l) #f)
     (super-instantiate ())))


... which (in 301) produces this error message:

class*: superclass method for override, overment, or rename-super is  
not overrideable: can-insert? for class: append-only-text%


I don't understand.

- dr