diff --git a/shape.c b/shape.c
index 4f003ad1d4..595a1a7ccc 100644
--- a/shape.c
+++ b/shape.c
@@ -757,16 +757,6 @@ rb_shape_has_object_id(rb_shape_t *shape)
     return shape->flags & SHAPE_FL_HAS_OBJECT_ID;
 }
 
-attr_index_t
-rb_shape_object_id_index(rb_shape_t *shape)
-{
-    RUBY_ASSERT(shape->flags & SHAPE_FL_HAS_OBJECT_ID);
-    while (shape->type != SHAPE_OBJ_ID) {
-        shape = rb_shape_get_parent(shape);
-    }
-    return shape->next_field_index - 1;
-}
-
 rb_shape_t *
 rb_shape_object_id_shape(VALUE obj)
 {
diff --git a/shape.h b/shape.h
index 1c0aa3c71a..4cb6c9d33d 100644
--- a/shape.h
+++ b/shape.h
@@ -173,7 +173,6 @@ shape_id_t rb_shape_transition_add_ivar_no_warnings(VALUE obj, ID id);
 
 rb_shape_t *rb_shape_object_id_shape(VALUE obj);
 bool rb_shape_has_object_id(rb_shape_t *shape);
-attr_index_t rb_shape_object_id_index(rb_shape_t *shape);
 void rb_shape_free_all(void);
 
 rb_shape_t *rb_shape_rebuild_shape(rb_shape_t *initial_shape, rb_shape_t *dest_shape);