diff --git a/README.md b/README.md
index 4cda4a65..173ad0f0 100644
--- a/README.md
+++ b/README.md
@@ -60,13 +60,16 @@ on Yjs. [![Become a Sponsor](https://img.shields.io/static/v1?label=Become%20a%2
   sharing analyses, documentation, spreadsheets, and dashboards.
 * [Nosgestesclimat](https://nosgestesclimat.fr/groupe) The french carbon
   footprint calculator has a group P2P mode based on yjs
+* [oorja.io](https://oorja.io) Online meeting spaces extensible with collaborative apps, end-to-end encrypted.
 * [LegendKeeper](https://legendkeeper.com) Collaborative campaign planner and worldbuilding app for tabletop RPGs.
 
+
 ## Table of Contents
 
 * [Overview](#Overview)
   * [Bindings](#Bindings)
   * [Providers](#Providers)
+  * [Ports](#Ports)
 * [Getting Started](#Getting-Started)
 * [API](#API)
   * [Shared Types](#Shared-Types)
@@ -154,6 +157,22 @@ y-websocket provider.
 </dd>
 </dl>
 
+# Ports
+
+There are several Yjs-compatible ports to other programming languages.
+
+* [y-octo](https://github.com/toeverything/y-octo) - Rust implementation by
+[AFFiNE](https://affine.pro)
+* [y-crdt](https://github.com/y-crdt/y-crdt) - Rust implementation with multiple
+language bindings to other languages
+  * [yrs](https://github.com/y-crdt/y-crdt/tree/main/yrs) - Rust interface
+  * [ypy](https://github.com/y-crdt/ypy) - Python binding
+  * [yrb](https://github.com/y-crdt/yrb) - Ruby binding
+  * [yrb](https://github.com/y-crdt/yswift) - Swift binding
+  * [yffi](https://github.com/y-crdt/y-crdt/tree/main/yffi) - C-FFI
+  * [ywasm](https://github.com/y-crdt/y-crdt/tree/main/ywasm) - WASM binding
+* [ycs](https://github.com/yjs/ycs) - .Net compatible C# implementation.
+
 ## Getting Started
 
 Install Yjs and a provider with your favorite package manager:
diff --git a/src/utils/UndoManager.js b/src/utils/UndoManager.js
index 746d0f01..f6f13ee5 100644
--- a/src/utils/UndoManager.js
+++ b/src/utils/UndoManager.js
@@ -15,6 +15,7 @@ import {
 
 import * as time from 'lib0/time'
 import * as array from 'lib0/array'
+import * as logging from 'lib0/logging'
 import { Observable } from 'lib0/observable'
 
 export class StackItem {
@@ -169,6 +170,7 @@ export class UndoManager extends Observable {
      * @type {Array<AbstractType<any>>}
      */
     this.scope = []
+    this.doc = doc
     this.addToScope(typeScope)
     this.deleteFilter = deleteFilter
     trackedOrigins.add(this)
@@ -189,7 +191,6 @@ export class UndoManager extends Observable {
      */
     this.undoing = false
     this.redoing = false
-    this.doc = doc
     this.lastChange = 0
     this.ignoreRemoteMapChanges = ignoreRemoteMapChanges
     this.captureTimeout = captureTimeout
@@ -263,6 +264,7 @@ export class UndoManager extends Observable {
     ytypes = array.isArray(ytypes) ? ytypes : [ytypes]
     ytypes.forEach(ytype => {
       if (this.scope.every(yt => yt !== ytype)) {
+        if (ytype.doc !== this.doc) logging.warn('[yjs#509] Not same Y.Doc') // use MultiDocUndoManager instead. also see https://github.com/yjs/yjs/issues/509
         this.scope.push(ytype)
       }
     })