diff --git a/docs/request-options.rst b/docs/request-options.rst
index 2a477aee9..4e5117b60 100644
--- a/docs/request-options.rst
+++ b/docs/request-options.rst
@@ -561,7 +561,7 @@ idn_conversion
 :Types:
     - bool
     - int
-:Default: ``true`` if ``intl`` extension is available, ``false`` otherwise
+:Default: ``true`` if ``intl`` extension is available (and ICU library is 4.6+ for PHP 7.2+), ``false`` otherwise
 :Constant: ``GuzzleHttp\RequestOptions::IDN_CONVERSION``
 
 .. code-block:: php
diff --git a/src/Client.php b/src/Client.php
index db4062f94..687358ed9 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -218,7 +218,8 @@ private function buildUri($uri, array $config)
         if ($uri->getHost() && isset($config['idn_conversion']) && ($config['idn_conversion'] !== false)) {
             $idnOptions = ($config['idn_conversion'] === true) ? IDNA_DEFAULT : $config['idn_conversion'];
 
-            $asciiHost = idn_to_ascii($uri->getHost(), $idnOptions, INTL_IDNA_VARIANT_UTS46, $info);
+            $idnaVariant = defined('INTL_IDNA_VARIANT_UTS46') ? INTL_IDNA_VARIANT_UTS46 : 0;
+            $asciiHost = idn_to_ascii($uri->getHost(), $idnOptions, $idnaVariant, $info);
             if ($asciiHost === false) {
                 $errorBitSet = isset($info['errors']) ? $info['errors'] : 0;
 
@@ -267,7 +268,14 @@ private function configureDefaults(array $config)
         ];
 
         // idn_to_ascii() is a part of ext-intl and might be not available
-        $defaults['idn_conversion'] = function_exists('idn_to_ascii');
+        $defaults['idn_conversion'] = function_exists('idn_to_ascii')
+            // Old ICU versions don't have this constant, so we are basically stuck (see https://github.com/guzzle/guzzle/pull/2424
+            // and https://github.com/guzzle/guzzle/issues/2448 for details)
+            && (
+                defined('INTL_IDNA_VARIANT_UTS46')
+                ||
+                PHP_VERSION_ID < 70200
+            );
 
         // Use the standard Linux HTTP_PROXY and HTTPS_PROXY if set.