diff --git a/lib/helpers/populate/getModelsMapForPopulate.js b/lib/helpers/populate/getModelsMapForPopulate.js
index 62fc180c6bc..abb68b7bf09 100644
--- a/lib/helpers/populate/getModelsMapForPopulate.js
+++ b/lib/helpers/populate/getModelsMapForPopulate.js
@@ -166,7 +166,7 @@ module.exports = function getModelsMapForPopulate(model, docs, options) {
         ret = localFieldPath.applyGetters(localFieldValue, hydratedDoc);
       }
     } else {
-      ret = convertTo_id(utils.getValue(localField, doc));
+      ret = convertTo_id(utils.getValue(localField, doc), schema);
     }
 
     const id = String(utils.getValue(foreignField, doc));
@@ -377,7 +377,7 @@ function handleRefFunction(ref, doc) {
  * @return {Array|Document|Any}
  */
 
-function convertTo_id(val) {
+function convertTo_id(val, schema) {
   if (val != null && val.$__ != null) return val._id;
 
   if (Array.isArray(val)) {
@@ -395,7 +395,11 @@ function convertTo_id(val) {
 
   // `populate('map')` may be an object if populating on a doc that hasn't
   // been hydrated yet
-  if (val != null && val.constructor.name === 'Object') {
+  if (val != null &&
+      val.constructor.name === 'Object' &&
+      // The intent here is we should only flatten the object if we expect
+      // to get a Map in the end. Avoid doing this for mixed types.
+      (schema == null || schema[schemaMixedSymbol] == null)) {
     const ret = [];
     for (const key of Object.keys(val)) {
       ret.push(val[key]);
diff --git a/test/types.map.test.js b/test/types.map.test.js
index 19cc6762a61..34a72d3ee04 100644
--- a/test/types.map.test.js
+++ b/test/types.map.test.js
@@ -371,7 +371,7 @@ describe('Map', function() {
       });
     });
 
-    it('avoid populating as map if populate on obj (gh-6460)', function() {
+    it('avoid populating as map if populate on obj (gh-6460) (gh-8157)', function() {
       const UserSchema = new mongoose.Schema({
         apiKeys: {}
       });
@@ -387,8 +387,12 @@ describe('Map', function() {
 
         const doc = yield User.create({ apiKeys: { github: key._id, twitter: key2._id } });
 
-        const populated = yield User.findById(doc).populate('apiKeys');
+        const populated = yield User.findById(doc).populate({
+          path: 'apiKeys',
+          skipInvalidIds: true
+        });
         assert.ok(!(populated.apiKeys instanceof Map));
+        assert.ok(!Array.isArray(populated.apiKeys));
       });
     });