httpi: 401 when authenticating via ntlm with domain

I’m trying to communicate with soap service via savon. It requires ntlm authentication with domain. I use following code to test service connection:

request = HTTPI::Request.new
request.url = "service_url"
request.auth.ntlm("username", "password", "domain")

response = HTTPI.get(request)
response.body

but always get following error

#<HTTPI::Response:0x00000002b5fe30 @code=401, @headers={"content-type"=>"text/html", "server"=>"Microsoft-IIS/7.0", "www-authenticate"=>["Negotiate", "NTLM"], "x-powered-by"=>"ASP.NET", "date"=>"Fri, 30 Aug 2013 06:26:58 GMT", "content-length"=>"1293"}

But, when I’m trying to access service via browser using Domain\Username and password everything is ok.

Am I doing smthing wrong??