diff --git a/src/mzscheme/src/struct.c b/src/mzscheme/src/struct.c
index 5e50001ce7..91a19306c7 100644
--- a/src/mzscheme/src/struct.c
+++ b/src/mzscheme/src/struct.c
@@ -1753,7 +1753,7 @@ static void chaperone_struct_set(const char *who, Scheme_Object *o, int i, Schem
 {
   while (1) {
     if (!SCHEME_CHAPERONEP(o)) {
-      SCHEME_VEC_ELS(o)[i] = v;
+      ((Scheme_Structure *)o)->slots[i] = v;
       return;
     } else {
       Scheme_Chaperone *px = (Scheme_Chaperone *)o;
diff --git a/src/mzscheme/src/vector.c b/src/mzscheme/src/vector.c
index 0213eb485a..f664cbaa55 100644
--- a/src/mzscheme/src/vector.c
+++ b/src/mzscheme/src/vector.c
@@ -364,7 +364,10 @@ void scheme_bad_vec_index(char *name, Scheme_Object *i, const char *what, Scheme
 static Scheme_Object *
 bad_index(char *name, Scheme_Object *i, Scheme_Object *vec, int bottom)
 {
-  scheme_bad_vec_index(name, i, "vector", vec, bottom, SCHEME_VEC_SIZE(vec));
+  scheme_bad_vec_index(name, i, "vector", vec, bottom, 
+                       (SCHEME_NP_CHAPERONEP(vec)
+                        ? SCHEME_VEC_SIZE(SCHEME_CHAPERONE_VAL(vec))
+                        : SCHEME_VEC_SIZE(vec)));
   return NULL;
 }
 
@@ -491,7 +494,7 @@ scheme_checked_vector_set(int argc, Scheme_Object *argv[])
   if (!SCHEME_MUTABLE_VECTORP(vec))
     scheme_wrong_type("vector-set!", "mutable vector", 0, argc, argv);
 
-  len = SCHEME_VEC_SIZE(argv[0]);
+  len = SCHEME_VEC_SIZE(vec);
 
   i = scheme_extract_index("vector-set!", 1, argc, argv, len, 0);