diff --git a/src/functions.php b/src/functions.php
index c352dc618..ee67f4437 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -336,7 +336,7 @@ function _current_time()
 function _idn_uri_convert(UriInterface $uri, $options = 0)
 {
     if ($uri->getHost()) {
-        $variant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : null;
+        $variant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : 0;
         $asciiHost = idn_to_ascii($uri->getHost(), $options, $variant, $info);
         if ($asciiHost === false) {
             $errorBitSet = isset($info['errors']) ? $info['errors'] : 0;