diff --git a/lib/helpers/isBsonType.js b/lib/helpers/isBsonType.js
index f75fd40169d..ab6ceba58e6 100644
--- a/lib/helpers/isBsonType.js
+++ b/lib/helpers/isBsonType.js
@@ -7,8 +7,7 @@
 
 function isBsonType(obj, typename) {
   return (
-    typeof obj === 'object' &&
-    obj !== null &&
+    obj != null &&
     obj._bsontype === typename
   );
 }
diff --git a/lib/helpers/schema/applyReadConcern.js b/lib/helpers/schema/applyReadConcern.js
index 80d4da6eb20..050fa9c6df0 100644
--- a/lib/helpers/schema/applyReadConcern.js
+++ b/lib/helpers/schema/applyReadConcern.js
@@ -1,7 +1,5 @@
 'use strict';
 
-const get = require('../get');
-
 module.exports = function applyReadConcern(schema, options) {
   if (options.readConcern !== undefined) {
     return;
@@ -15,7 +13,7 @@ module.exports = function applyReadConcern(schema, options) {
     return;
   }
 
-  const level = get(schema, 'options.readConcern.level', null);
+  const level = schema.options?.readConcern?.level;
   if (level != null) {
     options.readConcern = { level };
   }
diff --git a/lib/helpers/schema/applyWriteConcern.js b/lib/helpers/schema/applyWriteConcern.js
index 27098110872..28338cf58c3 100644
--- a/lib/helpers/schema/applyWriteConcern.js
+++ b/lib/helpers/schema/applyWriteConcern.js
@@ -1,7 +1,5 @@
 'use strict';
 
-const get = require('../get');
-
 module.exports = function applyWriteConcern(schema, options) {
   if (options.writeConcern != null) {
     return;
@@ -12,7 +10,7 @@ module.exports = function applyWriteConcern(schema, options) {
   if (options && options.session && options.session.transaction) {
     return;
   }
-  const writeConcern = get(schema, 'options.writeConcern', {});
+  const writeConcern = schema.options.writeConcern ?? {};
   if (Object.keys(writeConcern).length != 0) {
     options.writeConcern = {};
     if (!('w' in options) && writeConcern.w != null) {
diff --git a/lib/query.js b/lib/query.js
index 6333c153c68..ff86fa9389a 100644
--- a/lib/query.js
+++ b/lib/query.js
@@ -2248,6 +2248,9 @@ Query.prototype.error = function error(err) {
  */
 
 Query.prototype._unsetCastError = function _unsetCastError() {
+  if (this._error == null) {
+    return;
+  }
   if (this._error != null && !(this._error instanceof CastError)) {
     return;
   }
@@ -2291,9 +2294,9 @@ Query.prototype.mongooseOptions = function(v) {
 
 Query.prototype._castConditions = function() {
   let sanitizeFilterOpt = undefined;
-  if (this.model != null && utils.hasUserDefinedProperty(this.model.db.options, 'sanitizeFilter')) {
+  if (this.model != null && this.model.db.options?.sanitizeFilter != null) {
     sanitizeFilterOpt = this.model.db.options.sanitizeFilter;
-  } else if (this.model != null && utils.hasUserDefinedProperty(this.model.base.options, 'sanitizeFilter')) {
+  } else if (this.model != null && this.model.base.options?.sanitizeFilter != null) {
     sanitizeFilterOpt = this.model.base.options.sanitizeFilter;
   } else {
     sanitizeFilterOpt = this._mongooseOptions.sanitizeFilter;
@@ -2536,13 +2539,12 @@ Query.prototype.collation = function(value) {
  * @api private
  */
 
-Query.prototype._completeOne = function(doc, res, callback) {
+Query.prototype._completeOne = function(doc, res, projection, callback) {
   if (!doc && !this.options.includeResultMetadata) {
     return callback(null, null);
   }
 
   const model = this.model;
-  const projection = clone(this._fields);
   const userProvidedFields = this._userProvidedFields || {};
   // `populate`, `lean`
   const mongooseOptions = this._mongooseOptions;
@@ -2643,7 +2645,7 @@ Query.prototype._findOne = async function _findOne() {
   // don't pass in the conditions because we already merged them in
   const doc = await this.mongooseCollection.findOne(this._conditions, options);
   return new Promise((resolve, reject) => {
-    this._completeOne(doc, null, (err, res) => {
+    this._completeOne(doc, null, options.projection, (err, res) => {
       if (err) {
         return reject(err);
       }
@@ -3238,7 +3240,7 @@ function completeOne(model, doc, res, options, fields, userProvidedFields, pop,
 
   function _init(err, casted) {
     if (err) {
-      return immediate(() => callback(err));
+      return callback(err);
     }
 
 
@@ -3251,12 +3253,12 @@ function completeOne(model, doc, res, options, fields, userProvidedFields, pop,
       } else {
         res.value = null;
       }
-      return immediate(() => callback(null, res));
+      return callback(null, res);
     }
     if (options.session != null) {
       casted.$session(options.session);
     }
-    immediate(() => callback(null, casted));
+    callback(null, casted);
   }
 }
 
@@ -3465,7 +3467,7 @@ Query.prototype._findOneAndUpdate = async function _findOneAndUpdate() {
   const doc = !options.includeResultMetadata ? res : res.value;
 
   return new Promise((resolve, reject) => {
-    this._completeOne(doc, res, (err, res) => {
+    this._completeOne(doc, res, options.projection, (err, res) => {
       if (err) {
         return reject(err);
       }
@@ -3561,7 +3563,7 @@ Query.prototype._findOneAndDelete = async function _findOneAndDelete() {
   const doc = !includeResultMetadata ? res : res.value;
 
   return new Promise((resolve, reject) => {
-    this._completeOne(doc, res, (err, res) => {
+    this._completeOne(doc, res, options.projection, (err, res) => {
       if (err) {
         return reject(err);
       }
@@ -3715,7 +3717,7 @@ Query.prototype._findOneAndReplace = async function _findOneAndReplace() {
 
   const doc = !includeResultMetadata ? res : res.value;
   return new Promise((resolve, reject) => {
-    this._completeOne(doc, res, (err, res) => {
+    this._completeOne(doc, res, options.projection, (err, res) => {
       if (err) {
         return reject(err);
       }