diff --git a/lib/Horde/String.php b/lib/Horde/String.php
index d2797bf..ff4f00a 100644
--- a/lib/Horde/String.php
+++ b/lib/Horde/String.php
@@ -199,7 +199,11 @@ public static function lower($string, $locale = false, $charset = null)
         if (!isset(self::$_lowers[$string])) {
             $language = setlocale(LC_CTYPE, 0);
             setlocale(LC_CTYPE, 'C');
-            self::$_lowers[$string] = strtolower($string);
+            if ($string === null) {
+                self::$_lowers[$string] = '';
+            } else {
+                self::$_lowers[$string] = strtolower($string);
+            }
             setlocale(LC_CTYPE, $language);
         }