Problems with Web Services

I’m doing a Web Services using SOAP, but when I click a button, the
browser shows this message:

undefined method `parent’ for
#<SOAP::Parser::ParseFrame::NodeContainer:0x715fd7c @node=nil>

And the error line is in => app/controllers/users_controller.rb:10:in

8| def alta
9| client =,
10| @service_output = client.alta(params[:file]) # Error
11| end

Anybody know why is the problem?

Thanks and greetings.