diff --git a/js/jquery-currency-input.js b/js/jquery-currency-input.js
index 841ad6c..bf0b9e7 100644
--- a/js/jquery-currency-input.js
+++ b/js/jquery-currency-input.js
@@ -19,7 +19,7 @@ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLI
   $ = jQuery;
 
   isNumber = function(obj) {
-    return (toString.call(obj)) === '[object Number]' && isFinite(obj);
+    return (Object.prototype.toString.call(obj)) === '[object Number]' && isFinite(obj);
   };
 
   toNumber = function(str) {
diff --git a/src/jquery-currency-input.coffee b/src/jquery-currency-input.coffee
index 91158c2..17f2992 100644
--- a/src/jquery-currency-input.coffee
+++ b/src/jquery-currency-input.coffee
@@ -16,7 +16,7 @@ $ = jQuery
 
 # From _.isFinite
 isNumber = (obj) ->
-  (toString.call obj) == '[object Number]' and isFinite(obj)
+  (Object.prototype.toString.call obj) == '[object Number]' and isFinite(obj)
 
 toNumber = (str) -> 
   str = str.toString().replace /[$,\s]/g, ''