diff --git a/racket/src/cs/rumble/prefab.ss b/racket/src/cs/rumble/prefab.ss
index b6010a8e80..d878e35a04 100644
--- a/racket/src/cs/rumble/prefab.ss
+++ b/racket/src/cs/rumble/prefab.ss
@@ -280,7 +280,7 @@
   (let* ([l (if parent-key+size
                 (prefab-key+size->prefab-key-tail parent-key+size)
                 '())]
-         [l (let ([mutables (immutables->mutables immutables fields-count)])
+         [l (let ([mutables (immutables->mutables immutables fields-count 0)])
               (if (fx= 0 (#%vector-length mutables))
                   l
                   (cons mutables l)))]
@@ -305,14 +305,17 @@
 (define (encode-prefab-key+count-as-symbol prefab-key+count)
   (string->symbol (chez:format "~a" prefab-key+count)))
 
-(define (immutables->mutables immutables init-count)
+(define (immutables->mutables immutables init-count auto-count)
   (vector->immutable-vector
    (list->vector
     (let loop ([i 0])
       (cond
-       [(= i init-count) null]
-       [(chez:member i immutables) (loop (add1 i))]
-       [else (cons i (loop (add1 i)))])))))
+       [(fx= i init-count) (let loop ([i 0])
+                             (cond
+                              [(fx= i auto-count) null]
+                              [else (cons (fx+ i init-count) (loop (fx+ i 1)))]))]
+       [(#%memv i immutables) (loop (fx+ 1 i))]
+       [else (cons i (loop (fx+ i 1)))])))))
 
 (define (mutables->immutables mutables init-count)
   (let loop ([i 0])
diff --git a/racket/src/cs/rumble/struct.ss b/racket/src/cs/rumble/struct.ss
index 0709f5db37..c7d3006553 100644
--- a/racket/src/cs/rumble/struct.ss
+++ b/racket/src/cs/rumble/struct.ss
@@ -508,7 +508,7 @@
                [all-immutables (if (integer? proc-spec)
                                    (cons proc-spec immutables)
                                    immutables)]
-               [mutables (immutables->mutables all-immutables init-count)])
+               [mutables (immutables->mutables all-immutables init-count auto-count)])
           (when (not parent-rtd*)
             (record-type-equal-procedure rtd default-struct-equal?)
             (record-type-hash-procedure rtd default-struct-hash))