adding type casting of $title argument in URL helper functions to a string.  A numeric 0 sent to these functions would evaluate if ($title == '') as TRUE, and type casting seems the more appropriate fix than simply using $title === '', since we're expecting and treating $title as a string.
diff --git a/system/helpers/url_helper.php b/system/helpers/url_helper.php
index c05bc20..01cd345 100644
--- a/system/helpers/url_helper.php
+++ b/system/helpers/url_helper.php
@@ -90,6 +90,8 @@
  */	

 function anchor($uri = '', $title = '', $attributes = '')

 {

+	$title = (string) $title;

+	

 	if ( ! is_array($uri))

 	{

 		$site_url = ( ! preg_match('!^\w+://!i', $uri)) ? site_url($uri) : $uri;

@@ -132,6 +134,8 @@
  */

 function anchor_popup($uri = '', $title = '', $attributes = FALSE)

 {	

+	$title = (string) $title;

+	

 	$site_url = ( ! preg_match('!^\w+://!i', $uri)) ? site_url($uri) : $uri;

 	

 	if ($title == '')

@@ -170,6 +174,8 @@
  */

 function mailto($email, $title = '', $attributes = '')

 {

+	$title = (string) $title;

+	

 	if ($title == "")

 	{

 		$title = $email;

@@ -195,6 +201,8 @@
  */

 function safe_mailto($email, $title = '', $attributes = '')

 {

+	$title = (string) $title;

+	

 	if ($title == "")

 	{

 		$title = $email;