diff --git a/lib/types/map.js b/lib/types/map.js
index b992259870b..8d67324ea29 100644
--- a/lib/types/map.js
+++ b/lib/types/map.js
@@ -1,6 +1,7 @@
 'use strict';
 
 const Mixed = require('../schema/mixed');
+const ObjectId = require('./objectid');
 const deepEqual = require('../utils').deepEqual;
 const get = require('../helpers/get');
 const handleSpreadDoc = require('../helpers/document/handleSpreadDoc');
@@ -41,6 +42,10 @@ class MongooseMap extends Map {
   }
 
   get(key, options) {
+    if (key instanceof ObjectId) {
+      key = key.toString();
+    }
+
     options = options || {};
     if (options.getters === false) {
       return super.get(key);
@@ -49,6 +54,10 @@ class MongooseMap extends Map {
   }
 
   set(key, value) {
+    if (key instanceof ObjectId) {
+      key = key.toString();
+    }
+
     checkValidKey(key);
     value = handleSpreadDoc(value);
 
@@ -107,6 +116,10 @@ class MongooseMap extends Map {
   }
 
   delete(key) {
+    if (key instanceof ObjectId) {
+      key = key.toString();
+    }
+
     this.set(key, undefined);
     super.delete(key);
   }