diff --git a/pkgs/racket-test-core/tests/racket/object.rktl b/pkgs/racket-test-core/tests/racket/object.rktl
index 31c6102336..c407287500 100644
--- a/pkgs/racket-test-core/tests/racket/object.rktl
+++ b/pkgs/racket-test-core/tests/racket/object.rktl
@@ -1939,7 +1939,7 @@
       (define q 1)
       (field [s 1])
       (super-new)))
-  
+
   (err/rt-test (new d%) (make-undefined-exn? 'z))
   (err/rt-test (new d!%) (make-undefined-exn? 'z))
   (err/rt-test (new e%) (make-undefined-exn? 'z))
diff --git a/racket/src/cs/rumble/impersonator.ss b/racket/src/cs/rumble/impersonator.ss
index 81158d7fcf..c8631932f5 100644
--- a/racket/src/cs/rumble/impersonator.ss
+++ b/racket/src/cs/rumble/impersonator.ss
@@ -56,7 +56,8 @@
              rtd)
         (let ([r (loop (impersonator-next v))])
           (when (eq? r unsafe-undefined)
-            (raise-unsafe-undefined 'struct-ref "undefined" "use" acc (impersonator-val v) (cdr key)))
+            (let ([abs-pos (fx+ (cdr key) (struct-type-parent-total*-count (car key)))])
+              (raise-unsafe-undefined 'struct-ref "undefined" "use" acc (impersonator-val v) abs-pos)))
           r)]
        [(impersonator? v)
         (loop (impersonator-next v))]
@@ -95,7 +96,7 @@
           (when (eq? (unsafe-struct*-ref (impersonator-val v) abs-pos) unsafe-undefined)
             (unless (eq? (continuation-mark-set-first #f prop:chaperone-unsafe-undefined)
                          unsafe-undefined)
-              (raise-unsafe-undefined 'struct-set! "assignment disallowed" "assign" set (impersonator-val v) pos)))
+              (raise-unsafe-undefined 'struct-set! "assignment disallowed" "assign" set (impersonator-val v) abs-pos)))
           (loop (impersonator-next v) a)]
          [(impersonator? v)
           (loop (impersonator-next v) a)]