diff --git a/shape.h b/shape.h
index 8df12d3fb1388c..60084a4aff7b93 100644
--- a/shape.h
+++ b/shape.h
@@ -216,7 +216,6 @@ rb_shape_t *rb_shape_traverse_from_new_root(rb_shape_t *initial_shape, rb_shape_
 bool rb_shape_set_shape_id(VALUE obj, shape_id_t shape_id);
 
 VALUE rb_obj_debug_shape(VALUE self, VALUE obj);
-VALUE rb_shape_flags_mask(void);
 void rb_shape_set_too_complex(VALUE obj);
 
 // For ext/objspace