1 files changed, 8 insertions, 1 deletions
diff --git a/shape.c b/shape.c
index 6d6235dd3d..96cd2750af 100644
--- a/shape.c
+++ b/shape.c
@@ -654,13 +654,20 @@ rb_shape_get_next_iv_shape(rb_shape_t* shape, ID id)
 }
 
 rb_shape_t *
-rb_shape_get_next(rb_shape_t* shape, VALUE obj, ID id)
+rb_shape_get_next(rb_shape_t *shape, VALUE obj, ID id)
 {
     RUBY_ASSERT(!is_instance_id(id) || RTEST(rb_sym2str(ID2SYM(id))));
     if (UNLIKELY(shape->type == SHAPE_OBJ_TOO_COMPLEX)) {
         return shape;
     }
 
+#if RUBY_DEBUG
+    attr_index_t index;
+    if (rb_shape_get_iv_index(shape, id, &index)) {
+        rb_bug("rb_shape_get_next: trying to create ivar that already exists at index %u", index);
+    }
+#endif
+
     bool allow_new_shape = true;
 
     if (BUILTIN_TYPE(obj) == T_OBJECT) {