diff --git a/lib/Horde/String.php b/lib/Horde/String.php
index 9027d26..51137b6 100644
--- a/lib/Horde/String.php
+++ b/lib/Horde/String.php
@@ -950,6 +950,9 @@ public static function validUtf8($text)
                 }
 
                 do {
+                    if ($i + 1 >= $len) {
+                        return false;
+                    }
                     $c = ord($text[++$i]);
                     if (($c < 128) || ($c > 191)) {
                         return false;
diff --git a/src/HordeString.php b/src/HordeString.php
index e6f9b56..55b64c4 100644
--- a/src/HordeString.php
+++ b/src/HordeString.php
@@ -915,6 +915,9 @@ public static function validUtf8($text)
                 }
 
                 do {
+                    if ($i + 1 >= $len) {
+                        return false;
+                    }
                     $c = ord($text[++$i]);
                     if (($c < 128) || ($c > 191)) {
                         return false;