API hit error in ror gives - Completed 500 Internal Server Error , TypeError (no implicit conversion of nil into String)

My routes.rb -

Rails.application.routes.draw do
  root 'welcome#index'

  namespace :api, defaults: { format: :json } do
namespace :v1 do
  get "/users/:cv_id" => "engine#users_get"
  post "/users" => "engine#users_add"
end
end

and my main controller is --

   class Api::V1::MainController < ApplicationController

 http_basic_authenticate_with name: ENV["API_USERNAME"], password: ENV["API_PASSWORD"]


 API_CMDS_LIST = {
"Engine"  => [:users_add, :users_update],
}

 include Api::V1::ErrorMessage
  skip_before_filter :verify_authenticity_token
  protect_from_forgery with: :null_session

  def resulting_json(resp)
    if resp[:result] == "error" && !resp[:code]
     resp[:code] = Api::V1::ErrorMessage::CODES[resp[:message]] || Api::V1::ErrorMessage::DEFAULT_CODE
end

render json: resp
 end

 def authenticate
    #authenticate_or_request_with_http_basic do |user, password|

      #File.write('test.txt', "USER: #{user}. PASS: #{password}")
         #@user && password == @user.secret_token #&&  @user.belongs_to_active_brand?
       true
      #end
    end

   def validate_request
      action = params[:action]
      klass = API_CMDS_LIST.select{ |klass, methods| methods.include?(action.to_sym) }.keys[0]

        @req = "Api::V1::#{klass}".constantize.new(send("# {action}_params").merge( {user: @user} ), request)

     if !@req.valid?
       err_msg = @req.errors.full_messages.first
       code = Api::V1::ErrorMessage::CODES[err_msg] || Api::V1::ErrorMessage::DEFAULT_CODE
      render json: { result: "error", msg: err_msg, code: code } and   return
     end
end

   end

and engine controller code is -

     class Api::V1::EngineController < Api::V1::MainController
    include Api::V1::ErrorMessage
    before_filter :validate_request



   API_CMDS_LIST["Engine"].each do |method|

    define_method(method) do
        response = @req.send(method)
           resulting_json(response)
    end
    end
   def users_add_params
        params.permit(:first_name,
              :last_name) 
  end
  end

but when i try to hit the endpoint "/users" with params {first_name : "satyam",last_name:"agrawal"} it gives me this error ---

Processing by Api::V1::EngineController#users_add as JSON Parameters: {"first_name"=>"satyam", "last_name"=>"agrawal"} Completed 500 Internal Server Error in 1ms (ActiveRecord: 0.0ms)

TypeError (no implicit conversion of nil into String):

activesupport (5.0.1) lib/active_support/security_utils.rb:23:in digest' activesupport (5.0.1) lib/active_support/security_utils.rb:23:inhexdigest' activesupport (5.0.1) lib/active_support/security_utils.rb:23:in variable_size_secure_compare' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:75:inblock (2 levels) in http_basic_authenticate_with' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:97:in call' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:97:inauthenticate' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:87:in authenticate_with_http_basic' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:83:inauthenticate_or_request_with_http_basic' actionpack (5.0.1) lib/action_controller/metal/http_authentication.rb:71:in block in http_basic_authenticate_with' activesupport (5.0.1) lib/active_support/callbacks.rb:396:ininstance_exec' activesupport (5.0.1) lib/active_support/callbacks.rb:396:in block in make_lambda' activesupport (5.0.1) lib/active_support/callbacks.rb:169:incall' activesupport (5.0.1) lib/active_support/callbacks.rb:169:in block (2 levels) in halting' actionpack (5.0.1) lib/abstract_controller/callbacks.rb:12:incall' actionpack (5.0.1) lib/abstract_controller/callbacks.rb:12:in block (2 levels) in <module:Callbacks>' activesupport (5.0.1) lib/active_support/callbacks.rb:170:incall' activesupport (5.0.1) lib/active_support/callbacks.rb:170:in block in halting' activesupport (5.0.1) lib/active_support/callbacks.rb:454:incall' activesupport (5.0.1) lib/active_support/callbacks.rb:454:in block in call' activesupport (5.0.1) lib/active_support/callbacks.rb:454:ineach' activesupport (5.0.1) lib/active_support/callbacks.rb:454:in call' activesupport (5.0.1) lib/active_support/callbacks.rb:101:inrun_callbacks' activesupport (5.0.1) lib/active_support/callbacks.rb:750:in _run_process_action_callbacks' activesupport (5.0.1) lib/active_support/callbacks.rb:90:inrun_callbacks' actionpack (5.0.1) lib/abstract_controller/callbacks.rb:19:in process_action' actionpack (5.0.1) lib/action_controller/metal/rescue.rb:20:inprocess_action' actionpack (5.0.1) lib/action_controller/metal/instrumentation.rb:32:in block in process_action' activesupport (5.0.1) lib/active_support/notifications.rb:164:inblock in instrument' activesupport (5.0.1) lib/active_support/notifications/instrumenter.rb:21:in instrument' activesupport (5.0.1) lib/active_support/notifications.rb:164:ininstrument' actionpack (5.0.1) lib/action_controller/metal/instrumentation.rb:30:in process_action' actionpack (5.0.1) lib/action_controller/metal/params_wrapper.rb:248:inprocess_action' activerecord (5.0.1) lib/active_record/railties/controller_runtime.rb:18:in process_action' actionpack (5.0.1) lib/abstract_controller/base.rb:126:inprocess' actionview (5.0.1) lib/action_view/rendering.rb:30:in process' actionpack (5.0.1) lib/action_controller/metal.rb:190:indispatch' actionpack (5.0.1) lib/action_controller/metal.rb:262:in dispatch' actionpack (5.0.1) lib/action_dispatch/routing/route_set.rb:50:indispatch' actionpack (5.0.1) lib/action_dispatch/routing/route_set.rb:32:in serve' actionpack (5.0.1) lib/action_dispatch/journey/router.rb:39:inblock in serve' actionpack (5.0.1) lib/action_dispatch/journey/router.rb:26:in each' actionpack (5.0.1) lib/action_dispatch/journey/router.rb:26:inserve' actionpack (5.0.1) lib/action_dispatch/routing/route_set.rb:725:in call' rack (2.0.1) lib/rack/etag.rb:25:incall' rack (2.0.1) lib/rack/conditional_get.rb:38:in call' rack (2.0.1) lib/rack/head.rb:12:incall' rack (2.0.1) lib/rack/session/abstract/id.rb:222:in context' rack (2.0.1) lib/rack/session/abstract/id.rb:216:incall' actionpack (5.0.1) lib/action_dispatch/middleware/cookies.rb:613:in call' activerecord (5.0.1) lib/active_record/migration.rb:553:incall' actionpack (5.0.1) lib/action_dispatch/middleware/callbacks.rb:38:in block in call' activesupport (5.0.1) lib/active_support/callbacks.rb:97:inrun_callbacks' activesupport (5.0.1) lib/active_support/callbacks.rb:750:in _run_call_callbacks' activesupport (5.0.1) lib/active_support/callbacks.rb:90:inrun_callbacks' actionpack (5.0.1) lib/action_dispatch/middleware/callbacks.rb:36:in call' actionpack (5.0.1) lib/action_dispatch/middleware/executor.rb:12:incall' actionpack (5.0.1) lib/action_dispatch/middleware/remote_ip.rb:79:in call' actionpack (5.0.1) lib/action_dispatch/middleware/debug_exceptions.rb:49:incall' web-console (3.4.0) lib/web_console/middleware.rb:135:in call_app' web-console (3.4.0) lib/web_console/middleware.rb:28:inblock in call' web-console (3.4.0) lib/web_console/middleware.rb:18:in catch' web-console (3.4.0) lib/web_console/middleware.rb:18:incall' actionpack (5.0.1) lib/action_dispatch/middleware/show_exceptions.rb:31:in call' railties (5.0.1) lib/rails/rack/logger.rb:36:incall_app' railties (5.0.1) lib/rails/rack/logger.rb:24:in block in call' activesupport (5.0.1) lib/active_support/tagged_logging.rb:69:inblock in tagged' activesupport (5.0.1) lib/active_support/tagged_logging.rb:26:in tagged' activesupport (5.0.1) lib/active_support/tagged_logging.rb:69:intagged' railties (5.0.1) lib/rails/rack/logger.rb:24:in call' sprockets-rails (3.2.0) lib/sprockets/rails/quiet_assets.rb:13:incall' actionpack (5.0.1) lib/action_dispatch/middleware/request_id.rb:24:in call' rack (2.0.1) lib/rack/method_override.rb:22:incall' rack (2.0.1) lib/rack/runtime.rb:22:in call' activesupport (5.0.1) lib/active_support/cache/strategy/local_cache_middleware.rb:28:incall' actionpack (5.0.1) lib/action_dispatch/middleware/executor.rb:12:in call' actionpack (5.0.1) lib/action_dispatch/middleware/static.rb:136:incall' rack (2.0.1) lib/rack/sendfile.rb:111:in call' railties (5.0.1) lib/rails/engine.rb:522:incall' puma (3.7.1) lib/puma/configuration.rb:232:in call' puma (3.7.1) lib/puma/server.rb:578:inhandle_request' puma (3.7.1) lib/puma/server.rb:415:in process_client' puma (3.7.1) lib/puma/server.rb:275:inblock in run' puma (3.7.1) lib/puma/thread_pool.rb:120:in call' puma (3.7.1) lib/puma/thread_pool.rb:120:inblock in spawn_thread'

please help me to sort it out