diff --git a/lib/model.js b/lib/model.js
index c2786efc4af..e33e8d54452 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -968,18 +968,16 @@ Model.init = function init(callback) {
   const Promise = PromiseProvider.get();
   const autoIndex = this.schema.options.autoIndex;
   this.$init = new Promise((resolve, reject) => {
-    process.nextTick(() => {
-      if (autoIndex || (autoIndex == null && this.db.config.autoIndex)) {
-        this.ensureIndexes({ _automatic: true }, function(error) {
-          if (error) {
-            return reject(error);
-          }
-          resolve(this);
-        });
-      } else {
+    if (autoIndex || (autoIndex == null && this.db.config.autoIndex)) {
+      this.ensureIndexes({ _automatic: true }, function(error) {
+        if (error) {
+          return reject(error);
+        }
         resolve(this);
-      }
-    });
+      });
+    } else {
+      resolve(this);
+    }
   });
 
   if (callback) {
@@ -1195,6 +1193,7 @@ Model.createIndexes = function createIndexes(options, callback) {
 
 function _ensureIndexes(model, options, callback) {
   const indexes = model.schema.indexes();
+
   options = options || {};
 
   const done = function(err) {
@@ -1205,6 +1204,15 @@ function _ensureIndexes(model, options, callback) {
     callback && callback(err);
   };
 
+  for (const index of indexes) {
+    if (index[0]._id != null) {
+      const err = new Error('Cannot specify a custom index on `_id`, ' +
+        'MongoDB does not allow overwriting the default `_id` index. See ' +
+        'http://bit.ly/mongodb-id-index');
+      return done(err);
+    }
+  }
+
   if (!indexes.length) {
     utils.immediate(function() {
       done();
diff --git a/lib/schema.js b/lib/schema.js
index 8365db3381d..5143b0904fe 100644
--- a/lib/schema.js
+++ b/lib/schema.js
@@ -4,16 +4,17 @@
  * Module dependencies.
  */
 
-var readPref = require('./drivers').ReadPreference;
-var EventEmitter = require('events').EventEmitter;
-var VirtualType = require('./virtualtype');
-var utils = require('./utils');
-var MongooseTypes;
-var Kareem = require('kareem');
-var SchemaType = require('./schematype');
-var get = require('lodash.get');
-var getIndexes = require('./helpers/schema/getIndexes');
-var mpath = require('mpath');
+const EventEmitter = require('events').EventEmitter;
+const Kareem = require('kareem');
+const SchemaType = require('./schematype');
+const VirtualType = require('./virtualtype');
+const get = require('lodash.get');
+const getIndexes = require('./helpers/schema/getIndexes');
+const mpath = require('mpath');
+const readPref = require('./drivers').ReadPreference;
+const utils = require('./utils');
+
+let MongooseTypes;
 
 /**
  * Schema constructor.
@@ -509,38 +510,36 @@ Schema.prototype.path = function(path, obj) {
   branch[last] = utils.clone(obj);
 
   this.paths[path] = Schema.interpretAsType(path, obj, this.options);
+  const schemaType = this.paths[path];
 
-  if (this.paths[path].$isSchemaMap) {
+  if (schemaType.$isSchemaMap) {
     // Maps can have arbitrary keys, so `$*` is internal shorthand for "any key"
     // The '$' is to imply this path should never be stored in MongoDB so we
     // can easily build a regexp out of this path, and '*' to imply "any key."
     const mapPath = path + '.$*';
     this.paths[path + '.$*'] = Schema.interpretAsType(mapPath,
       obj.of || { type: {} }, this.options);
-    this.paths[path].$__schemaType = this.paths[path + '.$*'];
+    schemaType.$__schemaType = this.paths[path + '.$*'];
   }
 
-  if (this.paths[path].$isSingleNested) {
-    for (let key in this.paths[path].schema.paths) {
-      this.singleNestedPaths[path + '.' + key] =
-          this.paths[path].schema.paths[key];
+  if (schemaType.$isSingleNested) {
+    for (let key in schemaType.schema.paths) {
+      this.singleNestedPaths[path + '.' + key] = schemaType.schema.paths[key];
     }
-    for (let key in this.paths[path].schema.singleNestedPaths) {
+    for (let key in schemaType.schema.singleNestedPaths) {
       this.singleNestedPaths[path + '.' + key] =
-          this.paths[path].schema.singleNestedPaths[key];
+        schemaType.schema.singleNestedPaths[key];
     }
 
     this.childSchemas.push({
-      schema: this.paths[path].schema,
-      model: this.paths[path].caster
+      schema: schemaType.schema,
+      model: schemaType.caster
     });
-  } else if (this.paths[path].$isMongooseDocumentArray) {
+  } else if (schemaType.$isMongooseDocumentArray) {
     this.childSchemas.push({
-      schema: this.paths[path].schema,
-      model: this.paths[path].casterConstructor
+      schema: schemaType.schema,
+      model: schemaType.casterConstructor
     });
-  } else if (this.paths[path].path === '_id' && this.paths[path].options && this.paths[path].options.unique) {
-    throw new Error('Cannot put unique index on _id');
   }
 
   return this;
diff --git a/test/model.indexes.test.js b/test/model.indexes.test.js
index b5952ad5f9c..2989247474d 100644
--- a/test/model.indexes.test.js
+++ b/test/model.indexes.test.js
@@ -139,7 +139,7 @@ describe('model', function() {
 
     it('of multiple embedded documents with same schema', function(done) {
       var BlogPosts = new Schema({
-        _id: {type: ObjectId, index: true},
+        _id: {type: ObjectId, unique: true},
         title: {type: String, index: true},
         desc: String
       });
diff --git a/test/schema.test.js b/test/schema.test.js
index 8029d1b5e97..af3f1d4b4b4 100644
--- a/test/schema.test.js
+++ b/test/schema.test.js
@@ -835,17 +835,6 @@ describe('schema', function() {
 
         done();
       });
-      it('errors if unique index on _id', function(done) {
-        try {
-          var schema = new Schema({ _id: { type: mongoose.Schema.Types.ObjectId, unique: true } });
-
-          schema.path('_id').index({ unique: true });
-          done(new Error('Should not have reached this point!'));
-        } catch (error) {
-          assert.equal(error.message, 'Cannot put unique index on _id');
-          done();
-        }
-      });
 
       it('with single nested doc (gh-6113)', function(done) {
         var pointSchema = new Schema({