[racket] struct->vector interaction with BSL structures

Suppose I have these definitions in a BSL file:
  (define-struct fighter (desig accel speed range))
  (define fighter1 (make-fighter "F22" 75 200 350))

Now, in a separate 'teachpack' (written in #lang racket), I have an
exported function that processes the 'fighter1' value. But why does
struct->vector applied to the structure give something like this:
   #(struct:fighter "F22" 75 200 350 #f)
i.e. what is that #f doing at the end?

--- nadeem