diff --git a/lib/common.lib.php b/lib/common.lib.php
index 18562a637..b6bb9837c 100644
--- a/lib/common.lib.php
+++ b/lib/common.lib.php
@@ -2219,7 +2219,10 @@ function get_checked($field, $value)
 
 function is_mobile()
 {
-    return preg_match('/'.G5_MOBILE_AGENT.'/i', $_SERVER['HTTP_USER_AGENT']);
+    if (isset($_SERVER['HTTP_USER_AGENT']))
+        return  preg_match('/'.G5_MOBILE_AGENT.'/i', $_SERVER['HTTP_USER_AGENT']);
+    else
+        return '';
 }