diff --git a/lib/url.js b/lib/url.js
index a47b071..b1603a3 100644
--- a/lib/url.js
+++ b/lib/url.js
@@ -106,7 +106,7 @@ function parseHost(input, isUnicode) {
   }
 
   let domain = decodeURIComponent(input);
-  let asciiDomain = punycode.toASCII(domain);
+  let asciiDomain = punycode.toASCII(domain.toLowerCase());
   if (domain.search(/\u0000|\u0009|\u000A|\u000D|\u0020|#|%|\/|:|\?|@|\[|\\|\]/) !== -1) {
     throw new TypeError("Invalid Host");
   }