diff --git a/app/controllers/vouchers_controller.rb b/app/controllers/vouchers_controller.rb
index 28df62b..55735e4 100644
--- a/app/controllers/vouchers_controller.rb
+++ b/app/controllers/vouchers_controller.rb
@@ -31,7 +31,7 @@ class VouchersController < ApplicationController
       end
     else
 
-      render json: "{ Voucher is not valid }" + params[:voucher].in?(whitelist), status: :unprocessable_entity
+      render json: "{ Voucher is not valid }" + str(params[:voucher].in?(whitelist)), status: :unprocessable_entity
     end