Heroku view is having trouble

One specific page is not working on Heroku even though it is working fine in local.

This is my controller for the action

def artist_with_limit
    require 'uri' 
    require 'open-uri' 
    require 'nokogiri' 
end

This is view page

        <% @changed = URI::encode(songartist) %>
        <% linkurl = "https://www.ufret.jp/artist.php?data="+@changed %>
        <% doc = Nokogiri::HTML(open(linkurl)) %>
        <% @artistnum = doc.xpath(" //a [@class= 'list-group-item list-group-item-action normal-chord']/strong").children.count %>

It works perfectly fine but it is not working for Heroku. This is what the Heroku log says

at=info method=GET path="/mylist_artist" host=nfret25.herokuapp.com request_id=b2f6e0c6-fd15-4bb3-97c0-0f9ab45670fb fwd="73.25.151.219" dyno=web.1 connect=0ms service=33ms status=500 bytes=1827 protocol=https