diff --git a/lib/checks.js b/lib/checks.js
index 6f5123d..9845fc0 100644
--- a/lib/checks.js
+++ b/lib/checks.js
@@ -3,13 +3,15 @@
  * Licensed under the MIT license.
  */
 
+'use strict';
+
 var util = require('util');
 
 var errors = module.exports = require('./errors');
 
 function failCheck(ExceptionConstructor, callee, messageFormat, formatArgs) {
     messageFormat = messageFormat || '';
-    var message = util.format.apply(this, [messageFormat].concat(formatArgs));
+    var message = util.format.apply(util, [messageFormat].concat(formatArgs));
     var error = new ExceptionConstructor(message);
     Error.captureStackTrace(error, callee);
     throw error;
@@ -23,40 +25,40 @@ function failStateCheck(callee, message, formatArgs) {
     failCheck(errors.IllegalStateError, callee, message, formatArgs);
 }
 
-module.exports.checkArgument = function(value, message) {
+function checkArgument(value, message) {
     if (!value) {
-        failArgumentCheck(arguments.callee, message,
+        failArgumentCheck(checkArgument, message,
             Array.prototype.slice.call(arguments, 2));
     }
-};
+}
 
-module.exports.checkState = function(value, message) {
+function checkState(value, message) {
     if (!value) {
-        failStateCheck(arguments.callee, message,
+        failStateCheck(checkState, message,
             Array.prototype.slice.call(arguments, 2));
     }
-};
+}
 
-module.exports.checkIsDef = function(value, message) {
+function checkIsDef(value, message) {
     if (value !== undefined) {
         return value;
     }
 
-    failArgumentCheck(arguments.callee, message ||
+    failArgumentCheck(checkIsDef, message ||
         'Expected value to be defined but was undefined.',
         Array.prototype.slice.call(arguments, 2));
-};
+}
 
-module.exports.checkIsDefAndNotNull = function(value, message) {
+function checkIsDefAndNotNull(value, message) {
     // Note that undefined == null.
     if (value != null) {
         return value;
     }
 
-    failArgumentCheck(arguments.callee, message ||
+    failArgumentCheck(checkIsDefAndNotNull, message ||
         'Expected value to be defined and not null but got "' +
         typeOf(value) + '".', Array.prototype.slice.call(arguments, 2));
-};
+}
 
 // Fixed version of the typeOf operator which returns 'null' for null values
 // and 'array' for arrays.
@@ -73,22 +75,28 @@ function typeOf(value) {
 }
 
 function typeCheck(expect) {
-    return function(value, message) {
+    return function typeChecker(value, message) {
         var type = typeOf(value);
 
         if (type == expect) {
             return value;
         }
 
-        failArgumentCheck(arguments.callee, message ||
+        failArgumentCheck(typeChecker, message ||
             'Expected "' + expect + '" but got "' + type + '".',
             Array.prototype.slice.call(arguments, 2));
     };
 }
 
-module.exports.checkIsString = typeCheck('string');
-module.exports.checkIsArray = typeCheck('array');
-module.exports.checkIsNumber = typeCheck('number');
-module.exports.checkIsBoolean = typeCheck('boolean');
-module.exports.checkIsFunction = typeCheck('function');
-module.exports.checkIsObject = typeCheck('object');
+module.exports = {
+    checkArgument: checkArgument,
+    checkState: checkState,
+    checkIsDef: checkIsDef,
+    checkIsDefAndNotNull: checkIsDefAndNotNull,
+    checkIsString: typeCheck('string'),
+    checkIsArray: typeCheck('array'),
+    checkIsNumber: typeCheck('number'),
+    checkIsBoolean: typeCheck('boolean'),
+    checkIsFunction: typeCheck('function'),
+    checkIsObject: typeCheck('object')
+};
diff --git a/lib/errors.js b/lib/errors.js
index dee7e7c..8d5f6d8 100644
--- a/lib/errors.js
+++ b/lib/errors.js
@@ -2,6 +2,7 @@
  * Copyright (c) 2012 Mathieu Turcotte
  * Licensed under the MIT license.
  */
+ 'use strict';
 
 var util = require('util');
 
@@ -22,4 +23,4 @@ util.inherits(IllegalStateError, Error);
 IllegalStateError.prototype.name = 'IllegalStateError';
 
 module.exports.IllegalStateError = IllegalStateError;
-module.exports.IllegalArgumentError = IllegalArgumentError;
\ No newline at end of file
+module.exports.IllegalArgumentError = IllegalArgumentError;