diff --git a/packages/mongo/collection/collection.js b/packages/mongo/collection/collection.js
index 3ac681cd8e..f41f97a68c 100644
--- a/packages/mongo/collection/collection.js
+++ b/packages/mongo/collection/collection.js
@@ -3,7 +3,8 @@ import { AsyncMethods } from './methods_async';
 import { SyncMethods } from './methods_sync';
 import { IndexMethods } from './methods_index';
 import {
-  ID_GENERATORS, normalizeOptions,
+  ID_GENERATORS,
+  normalizeOptions,
   setupAutopublish,
   setupConnection,
   setupDriver,
@@ -266,5 +267,3 @@ Meteor.Collection = Mongo.Collection;
 
 // Allow deny stuff is now in the allow-deny package
 Object.assign(Mongo.Collection.prototype, AllowDeny.CollectionPrototype);
-
-
diff --git a/packages/mongo/collection/collection_utils.js b/packages/mongo/collection/collection_utils.js
index a0f7442ded..e6c79646d5 100644
--- a/packages/mongo/collection/collection_utils.js
+++ b/packages/mongo/collection/collection_utils.js
@@ -88,12 +88,17 @@ export function normalizeOptions(options) {
     options.connection = options.manager;
   }
 
+  const cleanedOptions = Object.fromEntries(
+    Object.entries(options || {}).filter(([_, v]) => v !== undefined),
+  );
+
+  // 2) Spread defaults first, then only the defined overrides
   return {
     connection: undefined,
     idGeneration: 'STRING',
     transform: null,
     _driver: undefined,
     _preventAutopublish: false,
-    ...options,
+    ...cleanedOptions,
   };
 }
diff --git a/packages/mongo/tests/collection_tests.js b/packages/mongo/tests/collection_tests.js
index a0e9e7fecf..e8841c5892 100644
--- a/packages/mongo/tests/collection_tests.js
+++ b/packages/mongo/tests/collection_tests.js
@@ -485,3 +485,58 @@ Meteor.isServer && Tinytest.addAsync('collection - simple add', async function(t
   test.equal((await collection.findOneAsync(id)).a, 2);
   await collection.removeAsync({});
 });
+
+Tinytest.addAsync('collection - default idGeneration when not provided', async function(test) {
+  if (!Meteor.isServer) {
+    return;
+  }
+  // Create a collection without specifying idGeneration option
+  var collectionName = 'defaultIdGeneration' + test.id;
+  var collection = new Mongo.Collection(collectionName, { idGeneration: undefined });
+
+  // Insert a document
+  var id = await collection.insertAsync({a: 1});
+
+  // Verify that the _id is a string
+  test.isTrue(typeof id === 'string', 'Document _id should be a string when no idGeneration option is provided');
+  test.isFalse(id instanceof Mongo.ObjectID, 'Document _id should not be a Mongo.ObjectID when no idGeneration option is provided');
+
+  // Clean up
+  await collection.removeAsync({});
+});
+
+Tinytest.addAsync('collection - compare default idGeneration with explicit idGeneration', async function(test) {
+  if (!Meteor.isServer) {
+    return;
+  }
+    // Create a collection without specifying idGeneration option
+  var defaultCollectionName = 'defaultIdGeneration2' + test.id;
+  var defaultCollection = new Mongo.Collection(defaultCollectionName, { idGeneration: undefined });
+
+  // Create a collection with explicit STRING idGeneration
+  var stringCollectionName = 'stringIdGeneration' + test.id;
+  var stringCollection = new Mongo.Collection(stringCollectionName, { idGeneration: 'STRING' });
+
+  // Create a collection with MONGO idGeneration
+  var mongoCollectionName = 'mongoIdGeneration' + test.id;
+  var mongoCollection = new Mongo.Collection(mongoCollectionName, { idGeneration: 'MONGO' });
+
+  // Insert documents
+  var defaultId = await defaultCollection.insertAsync({a: 1});
+  var stringId = await stringCollection.insertAsync({a: 1});
+  var mongoId = await mongoCollection.insertAsync({a: 1});
+
+  // Verify default behaves like STRING
+  test.isTrue(typeof defaultId === 'string', 'Default idGeneration should produce string IDs');
+  test.isTrue(typeof stringId === 'string', 'STRING idGeneration should produce string IDs');
+  test.isFalse(defaultId instanceof Mongo.ObjectID, 'Default idGeneration should not produce Mongo.ObjectID');
+  test.isFalse(stringId instanceof Mongo.ObjectID, 'STRING idGeneration should not produce Mongo.ObjectID');
+
+  // Verify MONGO produces ObjectIDs
+  test.isTrue(mongoId instanceof Mongo.ObjectID, 'MONGO idGeneration should produce Mongo.ObjectID');
+
+  // Clean up
+  await defaultCollection.removeAsync({});
+  await stringCollection.removeAsync({});
+  await mongoCollection.removeAsync({});
+});