diff --git a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-let.rkt b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-let.rkt
index 0667af7b47..67acac2ef5 100644
--- a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-let.rkt
+++ b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-let.rkt
@@ -86,9 +86,8 @@
                    (syntax-parse (cadr p)
                      #:literal-sets (kernel-literals)
                      [(#%plain-lambda params body ...)
-                      (define-values (unboxed boxed)
-                        (for/fold ([unboxed empty] [boxed empty])
-                                  ([param (in-syntax #'params)]
+                      (define unboxed-args
+                        (for/list ([param (in-syntax #'params)]
                                    [dom doms]
                                    [i (in-naturals)])
                           (cond
@@ -98,14 +97,14 @@
                                     #'(begin body ...)))
                              ;; we can unbox
                              (log-optimization "unboxed var -> table" arity-raising-opt-msg param)
-                             (values (cons i unboxed) boxed)]
-                            [else (values unboxed (cons i boxed))])))
+                             #t]
+                            [else #f])))
                       ;; can we unbox anything?
-                      (and (> (length unboxed) 0)
+                      (and (member #t unboxed-args)
                            ;; if so, add to the table of functions with
                            ;; unboxed params, so we can modify its call
                            ;; sites, its body and its header
-                           (add-unboxed-fun! fun-name unboxed boxed))]
+                           (add-unboxed-fun! fun-name unboxed-args))]
                      [_ #f])]
                   [_ #f])))))
           rest)))
diff --git a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-tables.rkt b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-tables.rkt
index 2cf5d5d745..20e26028dd 100644
--- a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-tables.rkt
+++ b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/optimizer/unboxed-tables.rkt
@@ -37,7 +37,11 @@
 ;; params first, then all imaginary parts, then all boxed arguments
 (define unboxed-funs-table (make-free-id-table))
 
-(define (add-unboxed-fun! fun-name unboxed boxed)
+(define (add-unboxed-fun! fun-name unboxed-args)
+  (define unboxed
+    (for/list ([i (in-naturals)] [unboxed? unboxed-args] #:when unboxed?) i))
+  (define boxed
+    (for/list ([i (in-naturals)] [unboxed? unboxed-args] #:unless unboxed?) i))
   (dict-set! unboxed-funs-table fun-name (list unboxed boxed)))
 
 (define-syntax-class unboxed-fun