diff --git a/websocket-sharp/Server/HttpServer.cs b/websocket-sharp/Server/HttpServer.cs
index 0928d2c8..672c09cd 100644
--- a/websocket-sharp/Server/HttpServer.cs
+++ b/websocket-sharp/Server/HttpServer.cs
@@ -161,9 +161,17 @@ namespace WebSocketSharp.Server
         throw new ArgumentException (msg, "url");
 
       var host = getHost (uri);
+
       var addr = host.ToIPAddress ();
-      if (!addr.IsLocal ())
-        throw new ArgumentException ("The host part isn't a local host name: " + url, "url");
+      if (addr == null) {
+        msg = "The host part could not be converted to an IP address.";
+        throw new ArgumentException (msg, "url");
+      }
+
+      if (!addr.IsLocal ()) {
+        msg = "The IP address of the host is not a local IP address.";
+        throw new ArgumentException (msg, "url");
+      }
 
       init (host, addr, uri.Port, uri.Scheme == "https");
     }