[plt-scheme] bug in stepper?

Using language Intermediate Student with lambda

stepping through

(define (build-number alon)
  (local ((define (somefunc x y)
            (+ x (* 10 y))))
    (foldl somefunc 0 alon)))

(build-number '( 4 5 3 9 4))

gives you an error - "find-special-name: couldn't find expanded name for 
list"