diff --git a/pkgs/racket-pkgs/racket-test/tests/racket/chaperone.rktl b/pkgs/racket-pkgs/racket-test/tests/racket/chaperone.rktl
index 8eb17cd9c1..9252a64fa2 100644
--- a/pkgs/racket-pkgs/racket-test/tests/racket/chaperone.rktl
+++ b/pkgs/racket-pkgs/racket-test/tests/racket/chaperone.rktl
@@ -1508,4 +1508,10 @@
 
 ;; ----------------------------------------
 
+(let ()
+  (struct s ([a #:mutable]))
+  (err/rt-test (impersonate-struct 5 set-s-a! (lambda (a b) b))))
+
+;; ----------------------------------------
+
 (report-errs)
diff --git a/racket/src/racket/src/struct.c b/racket/src/racket/src/struct.c
index 642c2cb06d..01f6b61328 100644
--- a/racket/src/racket/src/struct.c
+++ b/racket/src/racket/src/struct.c
@@ -5458,7 +5458,10 @@ static Scheme_Object *do_chaperone_struct(const char *name, int is_impersonator,
     if (SCHEME_CHAPERONEP(proc)) proc = SCHEME_CHAPERONE_VAL(proc);
     if (SCHEME_TRUEP(struct_setter_p(1, a))) {
       kind = "mutator";
-      offset = stype->num_slots;
+      if (stype)
+        offset = stype->num_slots;
+      else
+        offset = 0;
     } else if (SCHEME_TRUEP(struct_getter_p(1, a))) {
       kind = "accessor";
       offset = 0;