diff --git a/addons/mail/static/src/core/common/channel_member_service.js b/addons/mail/static/src/core/common/channel_member_service.js
index 1794ecef424..2ddc394de5a 100644
--- a/addons/mail/static/src/core/common/channel_member_service.js
+++ b/addons/mail/static/src/core/common/channel_member_service.js
@@ -39,8 +39,8 @@ export class ChannelMemberService {
                 channelId: memberData.persona.guest ? memberData.channel.id : null,
             });
         }
-        member.threadId = memberData.threadId ?? member.threadId ?? memberData.channel.id;
-        if (!member.thread) {
+        member.threadId = memberData.threadId ?? member.threadId ?? memberData.channel?.id;
+        if (member.threadId && !member.thread) {
             // this prevents cyclic dependencies between mail.thread and discuss.channel.member
             this.env.bus.trigger("mail.thread/insert", {
                 id: member.threadId,
@@ -50,7 +50,7 @@ export class ChannelMemberService {
         switch (command) {
             case "insert":
                 {
-                    if (!member.thread.channelMembers.includes(member)) {
+                    if (member.thread && !member.thread.channelMembers.includes(member)) {
                         member.thread.channelMembers.push(member);
                     }
                 }
@@ -59,7 +59,9 @@ export class ChannelMemberService {
                 removeFromArray(this.store.channelMembers, member);
             // eslint-disable-next-line no-fallthrough
             case "insert-and-unlink":
-                removeFromArray(member.thread.channelMembers, member);
+                if (member.thread) {
+                    removeFromArray(member.thread.channelMembers, member);
+                }
                 break;
         }
     }
diff --git a/addons/mail/static/tests/composer/composer_tests.js b/addons/mail/static/tests/composer/composer_tests.js
index f1dff6e6f8d..1a9f39fd44a 100644
--- a/addons/mail/static/tests/composer/composer_tests.js
+++ b/addons/mail/static/tests/composer/composer_tests.js
@@ -486,6 +486,27 @@ QUnit.test("leave command on channel", async (assert) => {
     assert.verifySteps(["You unsubscribed from general."]);
 });
 
+QUnit.test("Can handle leave notification from unknown member", async (assert) => {
+    const pyEnv = await startServer();
+    const userId = pyEnv["res.users"].create({ name: "Dobby" });
+    const partnerId = pyEnv["res.partner"].create({ name: "Dobby", user_ids: [userId] });
+    const channelId = pyEnv["discuss.channel"].create({
+        name: "general",
+        channel_member_ids: [
+            Command.create({ partner_id: pyEnv.currentPartnerId }),
+            Command.create({ partner_id: partnerId }),
+        ],
+    });
+    const { env, openDiscuss } = await start();
+    await openDiscuss(channelId);
+    await env.services.orm.call("discuss.channel", "action_unfollow", [channelId], {
+        context: { mockedUserId: userId },
+    });
+    await click("button[title='Show Member List']");
+    assert.containsOnce($, ".o-discuss-ChannelMember:contains(Mitchell)");
+    assert.containsNone($, ".o-discuss-ChannelMember:contains(Dobby)");
+});
+
 QUnit.test("leave command on chat", async (assert) => {
     const pyEnv = await startServer();
     const partnerId = pyEnv["res.partner"].create({ name: "Chuck Norris" });
diff --git a/addons/mail/static/tests/helpers/mock_server/models/discuss_channel.js b/addons/mail/static/tests/helpers/mock_server/models/discuss_channel.js
index cccc0602aad..3ad7c7287a7 100644
--- a/addons/mail/static/tests/helpers/mock_server/models/discuss_channel.js
+++ b/addons/mail/static/tests/helpers/mock_server/models/discuss_channel.js
@@ -91,7 +91,7 @@ patch(MockServer.prototype, "mail/models/discuss_channel", {
             return this._mockDiscussChannelCreateGroup(partners_to);
         }
         if (args.model === "discuss.channel" && args.method === "execute_command_leave") {
-            return this._mockDiscussChannelExecuteCommandLeave(args);
+            return this._mockDiscussChannelExecuteCommandLeave(args, args.kwargs?.context);
         }
         if (args.model === "discuss.channel" && args.method === "execute_command_who") {
             return this._mockDiscussChannelExecuteCommandWho(args);
@@ -218,9 +218,6 @@ patch(MockServer.prototype, "mail/models/discuss_channel", {
             ["channel_id", "in", ids],
             ["partner_id", "=", partnerId],
         ]);
-        const formattedChannelMember = this._mockDiscussChannelMember_DiscussChannelMemberFormat([
-            channelMember.id,
-        ])[0];
         if (!channelMember) {
             return true;
         }
@@ -241,7 +238,7 @@ patch(MockServer.prototype, "mail/models/discuss_channel", {
             this.pyEnv["bus.bus"]._sendone(channel, "mail.record/insert", {
                 Channel: {
                     id: channel.id,
-                    channelMembers: [["insert-and-unlink", formattedChannelMember]],
+                    channelMembers: [["insert-and-unlink", { id: channelMember.id }]],
                     memberCount: this.pyEnv["discuss.channel.member"].searchCount([
                         ["channel_id", "=", channel.id],
                     ]),
@@ -844,10 +841,10 @@ patch(MockServer.prototype, "mail/models/discuss_channel", {
      *
      * @private
      */
-    _mockDiscussChannelExecuteCommandLeave(args) {
+    _mockDiscussChannelExecuteCommandLeave(args, context = {}) {
         const channel = this.getRecords("discuss.channel", [["id", "in", args.args[0]]])[0];
         if (channel.channel_type === "channel") {
-            this._mockDiscussChannelActionUnfollow([channel.id]);
+            this._mockDiscussChannelActionUnfollow([channel.id], context);
         } else {
             this._mockDiscussChannelChannelPin([channel.id], false);
         }