diff --git a/signup/signup-handler.php b/signup/signup-handler.php
index e5a04c7..d831568 100644
--- a/signup/signup-handler.php
+++ b/signup/signup-handler.php
@@ -24,7 +24,7 @@ function add_ban_info($name, $email) {
 
 function starts_with($string, $prefix)
 {
-    return substr($string, 0, strlen($prefix)) === $prefix;
+    return mb_substr($string, 0, mb_strlen($prefix)) === $prefix;
 }
 
 function is_ssh_pubkey($string)