ensure prev-indent is an exact-nonnegative-integer

beautiful-racket-demo/jsonic-demo-2/indenter.rkt

@@ -11,7 +11,7 @@
   (cond
     [(not prev-line) 0]
     [else
-     (define prev-indent (line-indent tbox prev-line))
+     (define prev-indent (or (line-indent tbox prev-line) 0))
      (cond
        [(left-bracket?
          (line-first-visible-char tbox prev-line))