diff --git a/package.json b/package.json
index 6c63505..8ca2dc3 100644
--- a/package.json
+++ b/package.json
@@ -36,12 +36,14 @@
     "eslint-plugin-import": "^1.8.0",
     "eslint-plugin-jsx-a11y": "^1.2.2",
     "eslint-plugin-react": "^5.1.1",
-    "mocha": "^2.5.3",
     "gulp": "^3.9.1",
     "gulp-coveralls": "^0.1.4",
     "gulp-eslint": "^2.0.0",
     "gulp-istanbul": "^0.10.4",
     "gulp-mocha": "^2.2.0",
-    "run-sequence": "^1.2.1"
+    "mocha": "^2.5.3",
+    "run-sequence": "^1.2.1",
+    "sinon": "^1.17.4",
+    "sinon-chai": "^2.8.0"
   }
 }
diff --git a/test/index.spec.js b/test/index.spec.js
index 77d5dfa..71a9420 100644
--- a/test/index.spec.js
+++ b/test/index.spec.js
@@ -3,6 +3,10 @@
 const chai = require('chai');
 const assert = chai.assert;
 const stupidPassword = require('../lib');
+const sinon = require("sinon");
+const sinonChai = require("sinon-chai");
+const expect = chai.expect;
+chai.use(sinonChai);
 
 describe('stupidPassword.isStupid()', () => {
   it('should pass for a stupid password', () => {
@@ -39,6 +43,14 @@ describe('stupidPassword.isStupid()', () => {
     assert.isFalse(stupidPassword.check('Pass990ver'));
     assert.isFalse(stupidPassword.check('ummoinnerEmbassava33'));
   });
+
+  it("should call the String toLowerCase method", function () {
+    const lower = sinon.spy(String.prototype, 'toLowerCase');
+    stupidPassword.isStupid('password');
+    lower.restore();
+    sinon.assert.calledOnce(lower);
+    sinon.assert.calledOn(lower, 'password');
+  });
 });
 
 describe('stupidPassword.rateOfUsage()', () => {
@@ -59,4 +71,12 @@ describe('stupidPassword.rateOfUsage()', () => {
     });
     assert.isObject(stupidPassword.rateOfUsage('SpaceOdessey2001Clarke'));
   });
+
+  it("should call the String toLowerCase method", function () {
+    const lower = sinon.spy(String.prototype, 'toLowerCase');
+    stupidPassword.isStupid('football');
+    lower.restore();
+    sinon.assert.calledOnce(lower);
+    sinon.assert.calledOn(lower, 'football');
+  });
 });