Problem with Group Chat Inviter

Hey guys, i coded myself a little Group Chat Inviter in c#.
It worked fine until it tried to invite an user who’s name ended with an underscore. (Monoxide13_)
I’m actually not sure if it’s an issue with the function i’m using or with the api but since every single name before worked fine i guess it’s the api.

using (var wb = new WebClient())
	var data = new NameValueCollection();
	data["oauth_token"] = _oauth;
	data["username"] = username;
	data["irc_channel"] = _channel;

	var response = wb.UploadValues("", "POST", data);

The exception i get with the mentioned username is

The remote server returned an error: (400) Bad Request.

   at System.Net.WebClient.UploadValues(Uri address, String method, NameValueCollection data)
   at GroupChatInviter.ProgressForm.InviteThread() in c:\Users\Freddy\Documents\Visual Studio 2013\Projects\GroupChatInviter\GroupChatInviter\ProgressForm.cs:line 81

Any suggestions how to fix that?