[vim9] "Cannot index a special variable" when applying += to class member

[lifepillar]

Steps to reproduce

Source this script:

vim9script

class Foo
  this.x: number

  def Add(n: number)
    this.x += n
  enddef
endclass

var f =  Foo.new(1)
f.Add(41)
echo f.x

This results in E909: Cannot index a special variable. A workaround consists in changing the assignment inside Add() into this.x = this.x + n.

Expected behaviour

No error should be raised and the output should be 42.

Version of Vim

9.0.1198

Environment

macOS 13.1
Apple Terminal
xterm-256color
ZSH 5.8.1

Logs and stack traces

No response