diff --git a/src/index.php b/src/index.php
index 8ba5347..eec75e0 100644
--- a/src/index.php
+++ b/src/index.php
@@ -106,7 +106,7 @@ public static function getAuthCode($shared_secret, $time_offset = 0) {
 
         $code = '';
         for ($i = 0; $i < self::CODE_LENGTH; $i++) {
-            $code .= substr(self::CHARSET, $fullcode % strlen(self::CHARSET), 1);
+            $code .= substr(self::CHARSET, (int) $fullcode % strlen(self::CHARSET), 1);
             $fullcode /= strlen(self::CHARSET);
         }