diff --git a/src/Url.php b/src/Url.php
index 13557f5..e03a6d6 100644
--- a/src/Url.php
+++ b/src/Url.php
@@ -492,7 +492,7 @@ class Url {
   public static function get_absolute_url($relative_url=null) {
     if (!is_string($relative_url))
         $relative_url = self :: get_current_url();
-    if (self :: $public_root_url[0] == '/') {
+    if (self :: $public_root_url && self :: $public_root_url[0] == '/') {
         Log :: debug(
           "URL :: get_absolute_url($relative_url): configured public root URL is relative ".
           "(%s) => try to detect it from current request infos.",