diff --git a/internal/variable.h b/internal/variable.h
index 3933279633..6dec6a6759 100644
--- a/internal/variable.h
+++ b/internal/variable.h
@@ -57,7 +57,7 @@ VALUE rb_gvar_defined(ID);
 void rb_const_warn_if_deprecated(const rb_const_entry_t *, VALUE, ID);
 rb_shape_t * rb_grow_iv_list(VALUE obj);
 void rb_ensure_iv_list_size(VALUE obj, uint32_t len, uint32_t newsize);
-struct gen_ivtbl * rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize);
+struct gen_ivtbl *rb_ensure_generic_iv_list_size(VALUE obj, rb_shape_t *shape, uint32_t newsize);
 attr_index_t rb_obj_ivar_set(VALUE obj, ID id, VALUE val);
 MJIT_SYMBOL_EXPORT_END
 
diff --git a/variable.c b/variable.c
index 0fd4a80887..c409209117 100644
--- a/variable.c
+++ b/variable.c
@@ -66,7 +66,10 @@ static st_table *generic_iv_tbl_;
 struct ivar_update {
     struct gen_ivtbl *ivtbl;
     uint32_t iv_index;
-    rb_shape_t* shape;
+    uint32_t max_index;
+#if !SHAPE_IN_BASIC_FLAGS
+    rb_shape_t *shape;
+#endif
 };
 
 void
@@ -1018,7 +1021,7 @@ generic_ivar_update(st_data_t *k, st_data_t *v, st_data_t u, int existing)
         }
     }
     FL_SET((VALUE)*k, FL_EXIVAR);
-    ivtbl = gen_ivtbl_resize(ivtbl, ivup->shape->next_iv_index);
+    ivtbl = gen_ivtbl_resize(ivtbl, ivup->max_index);
     // Reinsert in to the hash table because ivtbl might be a newly resized chunk of memory
     *v = (st_data_t)ivtbl;
     ivup->ivtbl = ivtbl;
@@ -1281,7 +1284,10 @@ generic_ivar_set(VALUE obj, ID id, VALUE val)
         RUBY_ASSERT(index == (shape->next_iv_index - 1));
     }
 
+    ivup.max_index = shape->next_iv_index;
+#if !SHAPE_IN_BASIC_FLAGS
     ivup.shape = shape;
+#endif
 
     RB_VM_LOCK_ENTER();
     {
@@ -1382,15 +1388,22 @@ rb_ensure_iv_list_size(VALUE obj, uint32_t current_capacity, uint32_t new_capaci
 }
 
 struct gen_ivtbl *
-rb_ensure_generic_iv_list_size(VALUE obj, uint32_t newsize)
+rb_ensure_generic_iv_list_size(VALUE obj, rb_shape_t *shape, uint32_t newsize)
 {
     struct gen_ivtbl * ivtbl = 0;
 
     RB_VM_LOCK_ENTER();
     {
         if (UNLIKELY(!gen_ivtbl_get_unlocked(obj, 0, &ivtbl) || newsize > ivtbl->numiv)) {
-            ivtbl = gen_ivtbl_resize(ivtbl, newsize);
-            st_insert(generic_ivtbl_no_ractor_check(obj), (st_data_t)obj, (st_data_t)ivtbl);
+            struct ivar_update ivup = {
+                .iv_index = newsize - 1,
+                .max_index = newsize,
+#if !SHAPE_IN_BASIC_FLAGS
+                .shape = shape
+#endif
+            };
+            st_update(generic_ivtbl_no_ractor_check(obj), (st_data_t)obj, generic_ivar_update, (st_data_t)&ivup);
+            ivtbl = ivup.ivtbl;
             FL_SET_RAW(obj, FL_EXIVAR);
         }
     }
diff --git a/version.h b/version.h
index 5dac221817..a148cc6c3a 100644
--- a/version.h
+++ b/version.h
@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 1
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 41
+#define RUBY_PATCHLEVEL 42
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 8b1542bd1a..fb77b1f1cf 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1403,11 +1403,11 @@ vm_setivar_default(VALUE obj, ID id, VALUE val, shape_id_t dest_shape_id, attr_i
         shape_id_t source_shape_id = dest_shape->parent_id;
 
         if (shape_id == source_shape_id && dest_shape->edge_name == id && dest_shape->type == SHAPE_IVAR) {
-        ivtbl = rb_ensure_generic_iv_list_size(obj, index + 1);
+            ivtbl = rb_ensure_generic_iv_list_size(obj, dest_shape, index + 1);
 #if SHAPE_IN_BASIC_FLAGS
-        RBASIC_SET_SHAPE_ID(obj, dest_shape_id);
+            RBASIC_SET_SHAPE_ID(obj, dest_shape_id);
 #else
-        ivtbl->shape_id = dest_shape_id;
+            RUBY_ASSERT(ivtbl->shape_id == dest_shape_id);
 #endif
         }
         else {