diff --git a/packages/mongo/collection.js b/packages/mongo/collection.js
index 6a1a6906a4..9793e4a57d 100644
--- a/packages/mongo/collection.js
+++ b/packages/mongo/collection.js
@@ -429,12 +429,18 @@ Mongo.Collection.prototype.insert = function insert(doc, callback) {
     throw new Error("insert requires an argument");
   }
 
-  // Shallow-copy the document and possibly generate an ID
-  doc = _.extend({}, doc);
+  // Make a shallow clone of the document, preserving its prototype.
+  doc = Object.create(
+    Object.getPrototypeOf(doc),
+    Object.getOwnPropertyDescriptors(doc)
+  );
 
   if ('_id' in doc) {
-    if (!doc._id || !(typeof doc._id === 'string' || doc._id instanceof Mongo.ObjectID)) {
-      throw new Error("Meteor requires document _id fields to be non-empty strings or ObjectIDs");
+    if (! doc._id ||
+        ! (typeof doc._id === 'string' ||
+           doc._id instanceof Mongo.ObjectID)) {
+      throw new Error(
+        "Meteor requires document _id fields to be non-empty strings or ObjectIDs");
     }
   } else {
     let generateId = true;
diff --git a/packages/mongo/mongo_livedata_tests.js b/packages/mongo/mongo_livedata_tests.js
index 932b98caca..98ef9955a2 100644
--- a/packages/mongo/mongo_livedata_tests.js
+++ b/packages/mongo/mongo_livedata_tests.js
@@ -1507,7 +1507,9 @@ testAsyncMulti('mongo-livedata - document with a custom type, ' + idGeneration,
       Meteor.call('createInsecureCollection', this.collectionName, collectionOptions);
       Meteor.subscribe('c-' + this.collectionName, expect());
     }
-  }, function (test, expect) {
+  },
+
+  function (test, expect) {
     var self = this;
     self.coll = new Mongo.Collection(this.collectionName, collectionOptions);
     var docId;
@@ -1525,13 +1527,17 @@ testAsyncMulti('mongo-livedata - document with a custom type, ' + idGeneration,
       test.isTrue(inColl);
       inColl && test.equal(inColl.d.speak(), "woof");
     }));
-  }, function (test, expect) {
+  },
+
+  function (test, expect) {
     var self = this;
     self.coll.insert(new Dog("rover", "orange"), expect(function (err, id) {
       test.isTrue(err);
       test.isFalse(id);
     }));
-  }, function (test, expect) {
+  },
+
+  function (test, expect) {
     var self = this;
     self.coll.update(
       self.docId, new Dog("rover", "orange"), expect(function (err) {
diff --git a/packages/mongo/package.js b/packages/mongo/package.js
index 85c5a094fd..a765a8052f 100644
--- a/packages/mongo/package.js
+++ b/packages/mongo/package.js
@@ -9,7 +9,7 @@
 
 Package.describe({
   summary: "Adaptor for using MongoDB and Minimongo over DDP",
-  version: '1.2.2'
+  version: '1.2.3'
 });
 
 Npm.depends({