diff --git a/src/plugins/liveobjects/livemap.ts b/src/plugins/liveobjects/livemap.ts
index a76f63f22..3b67d41cb 100644
--- a/src/plugins/liveobjects/livemap.ts
+++ b/src/plugins/liveobjects/livemap.ts
@@ -9,10 +9,12 @@ import {
   Primitive,
   Value,
 } from '../../../liveobjects';
+import { DefaultInstance } from './instance';
 import { LiveCounter } from './livecounter';
 import { LiveCounterValueType } from './livecountervaluetype';
 import { LiveMapValueType } from './livemapvaluetype';
 import { LiveObject, LiveObjectData, LiveObjectUpdate, LiveObjectUpdateNoop } from './liveobject';
+import { DefaultPathObject } from './pathobject';
 import {
   getObjectDataPrimitive,
   MapRemove,
@@ -186,6 +188,19 @@ export class LiveMap<T extends Record<string, Value> = Record<string, Value>>
     ) {
       throw new client.ErrorInfo('Map value data type is unsupported', 40013, 400); // OD4a
     }
+
+    // RTLMV4c1 - live objects obtained from the channel, and the public objects that wrap them,
+    // are not valid map values. Without this check they would fall through to the JSON encoding
+    // branch and fail with a confusing serialization error (or leak internal state to the wire).
+    // To assign an object to a key, a new object must be created via the LiveMap.create() or
+    // LiveCounter.create() value types instead.
+    if (value instanceof LiveObject || value instanceof DefaultPathObject || value instanceof DefaultInstance) {
+      throw new client.ErrorInfo(
+        'Map value data type is unsupported: a reference to an existing object cannot be assigned as a map value; use LiveMap.create() or LiveCounter.create() to create a new object',
+        40013,
+        400,
+      );
+    }
   }
 
   /**
diff --git a/test/realtime/liveobjects.test.js b/test/realtime/liveobjects.test.js
index 21f62a8f3..cc13ee45f 100644
--- a/test/realtime/liveobjects.test.js
+++ b/test/realtime/liveobjects.test.js
@@ -4060,7 +4060,7 @@ define(['ably', 'shared_helper', 'chai', 'liveobjects', 'liveobjects_helper'], f
         {
           description: 'LiveMap.set throws on invalid input',
           action: async (ctx) => {
-            const { objectsHelper, channelName, entryInstance } = ctx;
+            const { objectsHelper, channelName, entryInstance, entryPathObject } = ctx;
 
             const mapCreatedPromise = waitForMapKeyUpdate(entryInstance, 'map');
             await objectsHelper.createAndSetOnMap(channelName, {
@@ -4086,6 +4086,12 @@ define(['ably', 'shared_helper', 'chai', 'liveobjects', 'liveobjects_helper'], f
             await expectToThrowAsync(async () => map.set('key', null), 'Map value data type is unsupported');
             await expectToThrowAsync(async () => map.set('key', BigInt(1)), 'Map value data type is unsupported');
             await expectToThrowAsync(async () => map.set('key', Symbol()), 'Map value data type is unsupported');
+
+            // RTLMV4c1 - references to existing objects (Instance or PathObject) are not valid
+            // map values; only LiveMap.create()/LiveCounter.create() value types can assign objects
+            await expectToThrowAsync(async () => map.set('key', map), 'Map value data type is unsupported');
+            await expectToThrowAsync(async () => map.set('key', entryInstance), 'Map value data type is unsupported');
+            await expectToThrowAsync(async () => map.set('key', entryPathObject), 'Map value data type is unsupported');
           },
         },