diff --git a/websocket-sharp/Net/HttpListenerPrefix.cs b/websocket-sharp/Net/HttpListenerPrefix.cs
index 1a449048..82268ddd 100644
--- a/websocket-sharp/Net/HttpListenerPrefix.cs
+++ b/websocket-sharp/Net/HttpListenerPrefix.cs
@@ -174,34 +174,29 @@ namespace WebSocketSharp.Net
         throw new ArgumentException (msg, "uriPrefix");
       }
 
-      var host = uriPrefix.IndexOf (':') + 3;
       var end = len - 1;
 
-      if (host > end) {
-        var msg = "No host is specified.";
+      if (uriPrefix[end] != '/') {
+        var msg = "It ends without '/'.";
 
         throw new ArgumentException (msg, "uriPrefix");
       }
 
-      if (uriPrefix[host] == ':') {
+      var host = uriPrefix.IndexOf (':') + 3;
+
+      if (host >= end) {
         var msg = "No host is specified.";
 
         throw new ArgumentException (msg, "uriPrefix");
       }
 
-      var root = uriPrefix.IndexOf ('/', host, len - host);
-
-      if (root == host) {
+      if (uriPrefix[host] == ':') {
         var msg = "No host is specified.";
 
         throw new ArgumentException (msg, "uriPrefix");
       }
 
-      if (uriPrefix[end] != '/') {
-        var msg = "It ends without '/'.";
-
-        throw new ArgumentException (msg, "uriPrefix");
-      }
+      var root = uriPrefix.IndexOf ('/', host, len - host);
 
       if (uriPrefix[root - 1] == ':') {
         var msg = "No port is specified.";