diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index 00bdf36..27184c8 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -28,7 +28,8 @@
 The fancy term is "object-oriented programming".  You can find lots of study
 material on this subject.  Here we document what |Vim9| script provides,
 assuming you know the basics already.  Added are helpful hints about how to
-use this functionality effectively.
+use this functionality effectively.  Vim9 classes and objects cannot be used
+in legacy Vim scripts and legacy functions.
 
 The basic item is an object:
 - An object stores state.  It contains one or more variables that can each