diff --git a/lib/grammar.js b/lib/grammar.js
index 2d73ccd..58c303f 100644
--- a/lib/grammar.js
+++ b/lib/grammar.js
@@ -18,10 +18,10 @@ Grammar = (function() {
   /* Private*/
 
   Grammar._toString = function(input) {
-    if (toString.call(input) === '[object String]') {
+    if (Object.prototype.toString.call(input) === '[object String]') {
       return input;
     }
-    if (toString.call(input) === '[object Array]') {
+    if (Object.prototype.toString.call(input) === '[object Array]') {
       return input.join('');
     }
     return '';
diff --git a/src/grammar.coffee b/src/grammar.coffee
index 0095516..5dc879c 100644
--- a/src/grammar.coffee
+++ b/src/grammar.coffee
@@ -23,8 +23,8 @@ class Grammar
   # method.
   #
   @_toString: (input) ->
-    return input if toString.call(input) is '[object String]'
-    return input.join('') if toString.call(input) is '[object Array]'
+    return input if Object.prototype.toString.call(input) is '[object String]'
+    return input.join('') if Object.prototype.toString.call(input) is '[object Array]'
     return ''