diff --git a/README.md b/README.md
index 0fba765..a07113b 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 This is a simple coupon creation project using NodeJS.
 
 [![license](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/yusufshakeel/couponjs)
-[![npm version](https://img.shields.io/badge/npm-0.8.15-blue.svg)](https://www.npmjs.com/package/couponjs)
+[![npm version](https://img.shields.io/badge/npm-0.8.16-blue.svg)](https://www.npmjs.com/package/couponjs)
 [![Build Status](https://travis-ci.com/yusufshakeel/couponjs.svg?branch=master)](https://travis-ci.com/yusufshakeel/couponjs)
 [![Coverage Status](https://coveralls.io/repos/github/yusufshakeel/couponjs/badge.svg?branch=master)](https://coveralls.io/github/yusufshakeel/couponjs?branch=master)
 
diff --git a/app/functional/index.js b/app/functional/index.js
index 054d156..16f38da 100644
--- a/app/functional/index.js
+++ b/app/functional/index.js
@@ -51,4 +51,20 @@ const omit = (values, valuesToOmit) => values.filter(value => !valuesToOmit.incl
  */
 const uniqueCharacters = characters => [...new Set(characters.join(''))];
 
-module.exports = { sumOf, attachPrefix, attachSuffix, pipe, identity, omit, uniqueCharacters };
+/**
+ * Copies all enumerable own properties from one or more objects to a empty target object.
+ * @param {...object} sourceObjects
+ * @returns {object}
+ */
+const shallowMerge = (...sourceObjects) => Object.assign({}, ...sourceObjects);
+
+module.exports = {
+  sumOf,
+  attachPrefix,
+  attachSuffix,
+  pipe,
+  identity,
+  omit,
+  uniqueCharacters,
+  shallowMerge
+};
diff --git a/index.js b/index.js
index 72fa2a8..5894ea5 100644
--- a/index.js
+++ b/index.js
@@ -5,6 +5,7 @@ const { defaultCouponGenerationOption, defaultCouponEngineOption } = require('./
 const randomInteger = require('./app/random-integer.js');
 const Performance = require('./app/performance.js');
 const { couponConfigValidator } = require('./app/validator/coupon-config-validator.js');
+const { shallowMerge } = require('./app/functional');
 
 /**
  * The Coupon constructor.
@@ -13,8 +14,7 @@ const { couponConfigValidator } = require('./app/validator/coupon-config-validat
  */
 const Coupon = function (config) {
   const performance = new Performance();
-  const { verbose, logPerformance, maxNumberOfCouponsToGenerate } = Object.assign(
-    {},
+  const { verbose, logPerformance, maxNumberOfCouponsToGenerate } = shallowMerge(
     defaultCouponEngineOption,
     config
   );
@@ -36,7 +36,7 @@ const Coupon = function (config) {
       omitCharacters,
       format,
       characterSet: characterSetOption
-    } = Object.assign({}, defaultCouponGenerationOption, option);
+    } = shallowMerge(defaultCouponGenerationOption, option);
     try {
       const engine = new Engine({
         randomInteger,
diff --git a/package-lock.json b/package-lock.json
index 42d6c1a..265b74b 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "couponjs",
-  "version": "0.8.15",
+  "version": "0.8.16",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
diff --git a/package.json b/package.json
index 8126545..dd6b4e0 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "couponjs",
-  "version": "0.8.15",
+  "version": "0.8.16",
   "description": "This is a simple coupon creation project using NodeJS.",
   "main": "index.js",
   "scripts": {
diff --git a/tests/unit/app/functional/index.unit.test.js b/tests/unit/app/functional/index.unit.test.js
index 0e4d96f..bf32bc5 100644
--- a/tests/unit/app/functional/index.unit.test.js
+++ b/tests/unit/app/functional/index.unit.test.js
@@ -7,7 +7,8 @@ const {
   pipe,
   identity,
   omit,
-  uniqueCharacters
+  uniqueCharacters,
+  shallowMerge
 } = require('../../../../app/functional');
 
 test('Should be able to sum up', () => {
@@ -54,3 +55,22 @@ test('Should be able to get unique characters', () => {
     'F'
   ]);
 });
+
+test('Should be able to shallow merge without mutating the source objects', () => {
+  const obj1 = { a: 1, b: 2, e: { f: [1, 2], g: { h: 10 } } };
+  const obj2 = { b: 3 };
+  const obj3 = { c: 4 };
+  const obj4 = { e: { f: 10 } };
+  expect(shallowMerge(obj1, obj2, obj3, obj4)).toStrictEqual({
+    a: 1,
+    b: 3,
+    c: 4,
+    e: {
+      f: 10
+    }
+  });
+  expect(obj1).toStrictEqual({ a: 1, b: 2, e: { f: [1, 2], g: { h: 10 } } });
+  expect(obj2).toStrictEqual({ b: 3 });
+  expect(obj3).toStrictEqual({ c: 4 });
+  expect(obj4).toStrictEqual({ e: { f: 10 } });
+});