diff --git a/lib/error-reporter.js b/lib/error-reporter.js
index 036a23d..cb2b955 100644
--- a/lib/error-reporter.js
+++ b/lib/error-reporter.js
@@ -2,8 +2,6 @@ var ErrorReporter,
   __bind = function(fn, me){ return function(){ return fn.apply(me, arguments); }; };
 
 ErrorReporter = (function() {
-  ErrorReporter.prototype._toString = Object.prototype.toString;
-
   ErrorReporter.prototype._sourceCode = null;
 
   function ErrorReporter(sourceCode) {
@@ -11,7 +9,7 @@ ErrorReporter = (function() {
     if (sourceCode == null) {
       throw new Error('Source code not provided');
     }
-    if (this._toString.call(sourceCode) !== '[object String]') {
+    if (Object.prototype.toString.call(sourceCode) !== '[object String]') {
       throw new TypeError('Source code must be a string');
     }
     this._sourceCode = sourceCode;
@@ -22,7 +20,7 @@ ErrorReporter = (function() {
     if (message == null) {
       throw new Error('Message not provided');
     }
-    if (this._toString.call(message) !== '[object String]') {
+    if (Object.prototype.toString.call(message) !== '[object String]') {
       throw new TypeError('Message must be a string');
     }
     if (message.length === 0) {
@@ -31,7 +29,7 @@ ErrorReporter = (function() {
     if (lineNumber == null) {
       throw new Error('Line number not provided');
     }
-    if (this._toString.call(lineNumber) !== '[object Number]') {
+    if (Object.prototype.toString.call(lineNumber) !== '[object Number]') {
       throw new TypeError('Line number must be a number');
     }
     if (lineNumber <= 0) {
@@ -40,7 +38,7 @@ ErrorReporter = (function() {
     if (columnNumber == null) {
       throw new Error('Column number not provided');
     }
-    if (this._toString.call(columnNumber) !== '[object Number]') {
+    if (Object.prototype.toString.call(columnNumber) !== '[object Number]') {
       throw new TypeError('Column number must be a number');
     }
     if (columnNumber <= 0) {
@@ -92,4 +90,4 @@ ErrorReporter = (function() {
 
 })();
 
-module.exports = ErrorReporter;
\ No newline at end of file
+module.exports = ErrorReporter;