diff --git a/collects/compiler/zo-marshal.rkt b/collects/compiler/zo-marshal.rkt
index a626ac7a7a..9438fc71aa 100644
--- a/collects/compiler/zo-marshal.rkt
+++ b/collects/compiler/zo-marshal.rkt
@@ -978,13 +978,14 @@
                        num-all-params
                        max-let-depth
                        (and tl-map
-                            (if (tl-map . < . #x7FFFFFFF)
+                            (if (tl-map . <= . #xFFFFFFF)
+                                ;; Encode as a fixnum:
                                 tl-map
                                 ;; Encode as an even-sized vector of 16-bit integers:
                                 (let ([len (* 2 (quotient (+ (integer-length tl-map) 31) 32))])
                                   (for/vector ([i (in-range len)])
-                                              (let ([s (* i 16)])
-                                                (bitwise-bit-field tl-map s (+ s 16)))))))
+                                    (let ([s (* i 16)])
+                                      (bitwise-bit-field tl-map s (+ s 16)))))))
                        name
                        l)
                       out))]))
diff --git a/collects/compiler/zo-parse.rkt b/collects/compiler/zo-parse.rkt
index ac27f36b56..66ee70a765 100644
--- a/collects/compiler/zo-parse.rkt
+++ b/collects/compiler/zo-parse.rkt
@@ -136,9 +136,9 @@
                    (and tl-map
                         (let* ([bits (if (exact-integer? tl-map)
                                          tl-map
-                                         (for/fold ([i 0]) ([v (in-list tl-map)]
+                                         (for/fold ([i 0]) ([v (in-vector tl-map)]
                                                             [s (in-naturals)])
-                                           (bitwise-ior i (arithmetic-shift v 16))))]
+                                           (bitwise-ior i (arithmetic-shift v (* s 16)))))]
                                [len (integer-length bits)])
                           (list->set
                            (let loop ([bit 0])