diff --git a/README.md b/README.md
index 43fc135..eb0a861 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,7 @@ redis-mock
 
 ![Build status](https://github.com/yeahoffline/redis-mock/workflows/Build/badge.svg?branch=master)
 ![node-current](https://img.shields.io/node/v/redis-mock)
-![mock-completeness](https://img.shields.io/badge/Methods%20mocked-46%25%20(222%2F482)-red)
+![mock-completeness](https://img.shields.io/badge/Methods%20mocked-46%25%20(224%2F482)-red)
 [![GitHub issues](https://img.shields.io/github/issues/yeahoffline/redis-mock)](https://github.com/yeahoffline/redis-mock/issues)
 
 The goal of the `redis-mock` project is to create a feature-complete mock of [node_redis](https://github.com/NodeRedis/node_redis), which may be used interchangeably when writing unit tests for code that depends on `redis`.
diff --git a/lib/server/strings.js b/lib/server/strings.js
index 29b1df6..a9b9d56 100644
--- a/lib/server/strings.js
+++ b/lib/server/strings.js
@@ -34,17 +34,20 @@ exports.set = function (key, value, callback, args = {}) {
   const keyExists = key in this.storage;
   const expirationTime = getExpirationTime();
 
+  let response = 'OK';
   if ((keyExists && args.xx) || (!keyExists && args.nx) || (!args.xx && !args.nx)) {
     // it it's okay to set the value
     this.storage[key] = createItem(value);
+  } else if (args.xx || args.nx) {
+    response = null;
   }
 
   if (expirationTime) {
     this.expire(key, expirationTime, (err, result) => {
-      helpers.callCallback(callback, err, "OK");
+      helpers.callCallback(callback, err, response);
     });
   } else {
-    helpers.callCallback(callback, null, "OK");
+    helpers.callCallback(callback, null, response);
   }
 };
 
diff --git a/package-lock.json b/package-lock.json
index 590c403..a63409f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,11 +1,11 @@
 {
   "name": "redis-mock",
-  "version": "0.56.3",
+  "version": "0.56.4",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
-      "version": "0.56.3",
+      "version": "0.56.4",
       "license": "MIT",
       "devDependencies": {
         "eslint": "^5.16.0",
diff --git a/package.json b/package.json
index 7a2a36b..f32bf6b 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "redis-mock",
-  "version": "0.56.3",
+  "version": "0.56.4",
   "description": "Redis client mock object for unit testing",
   "author": "Kristian Faeldt <kristian.faeldt@gmail.com>",
   "main": "./lib/index",