[racket] Stepper and application in BSL

I put the following in my definitions window (Beginning Student Language).

(define (f x)
  (+ x 1))

(+ (f 3) 1)

When I open the Stepper, (f 3) is highlighted in green, as expected.
If I click "Step >", the expression changes to (+ (+ 3 1) 1), as
expected (it stepped into the function).  However, if I instead click
on "Application >", I get "All of the definitions have been
successfully evaluated."  I expected to get (+ 4 1).  I thought the
stepper would replace (f 3) with the result of the application.  Why
didn't I see what I expected?

Justin