diff --git a/docs/content/guide/filter.ngdoc b/docs/content/guide/filter.ngdoc
index 8eedf14c8..9ef8bcc0f 100644
--- a/docs/content/guide/filter.ngdoc
+++ b/docs/content/guide/filter.ngdoc
@@ -103,6 +103,7 @@ text upper-case.
     angular.module('MyReverseModule', []).
       filter('reverse', function() {
         return function(input, uppercase) {
+          input = input || '';
           var out = "";
           for (var i = 0; i < input.length; i++) {
             out = input.charAt(i) + out;