diff --git a/lib/index.js b/lib/index.js
index 8fbe5dd..53cf7c7 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -7,6 +7,7 @@ function noQsMethod (options) {
   options        = xtend(defaults, options);
 
   return (socket) => {
+    'use strict'; // Node 4.x workaround
     const server = this.server || socket.server;
 
     if (!server.$emit) {
@@ -144,7 +145,9 @@ function authorize (options, onConnection) {
   const auth = xtend(defaults, options);
 
   return (socket, accept) => {
+    'use strict'; // Node 4.x workaround
     let token, error;
+
     const handshake = socket.handshake;
     const req = socket.request || socket;
     const authorization_header = (req.headers || {}).authorization;
@@ -219,6 +222,8 @@ function authorize (options, onConnection) {
 }
 
 function getSecret (request, secret, token, callback) {
+  'use strict'; // Node 4.x workaround
+
   if (typeof secret === 'function') {
     if (!token) {
       return callback({ code: 'invalid_token', message: 'jwt must be provided' });
diff --git a/test/fixture/index.js b/test/fixture/index.js
index 40a1384..2f77105 100644
--- a/test/fixture/index.js
+++ b/test/fixture/index.js
@@ -1,3 +1,5 @@
+'use strict'; // Node 4.x workaround
+
 const express = require('express');
 const http = require('http');
 
diff --git a/test/fixture/namespace.js b/test/fixture/namespace.js
index 09379e4..f8e5f8f 100644
--- a/test/fixture/namespace.js
+++ b/test/fixture/namespace.js
@@ -1,3 +1,5 @@
+'use strict'; // Node 4.x workaround
+
 const express = require('express');
 const http = require('http');
 
diff --git a/test/fixture/secret_function.js b/test/fixture/secret_function.js
index bd411cb..1ee17cd 100644
--- a/test/fixture/secret_function.js
+++ b/test/fixture/secret_function.js
@@ -1,3 +1,5 @@
+'use strict'; // Node 4.x workaround
+
 const express = require('express');
 const http = require('http');