title: TypeError: no implicit conversion of nil into String Heroku and Rails
body: <p>Okay, so I'm trying to deploy my app onto Heroku. </p>
  
  <p>Both of the following pieces of code result in success: </p>
  
  <pre><code>RAILS_ENV=production bundle exec rake assets:precompile
  rails assets:precompile
  </code></pre>
  
  <p>*It is frustrating that rails assets:precompile comes out successful individually but when trying to push it up to Heroku, it says that this command fails. </p>
  
  <p>Though when I commit those changes and try </p>
  
  <pre><code>git push heroku master
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>remote:        Running: rake assets:precompile
  remote:        rake aborted!
  remote:        TypeError: no implicit conversion of nil into String
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/config/initializers/devise.rb:21:in `+'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/config/initializers/devise.rb:21:in `block in &lt;top (required)&gt;'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/devise-4.5.0/lib/devise.rb:307:in `setup'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/config/initializers/devise.rb:5:in `&lt;top (required)&gt;'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:286:in `load'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:286:in `block in load'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:258:in `load_dependency'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:286:in `load'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/engine.rb:655:in `block in load_config_initializer'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/notifications.rb:168:in `instrument'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/engine.rb:654:in `load_config_initializer'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/engine.rb:612:in `block (2 levels) in &lt;class:Engine&gt;'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/engine.rb:611:in `each'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/engine.rb:611:in `block in &lt;class:Engine&gt;'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:30:in `instance_exec'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:30:in `run'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:59:in `block in run_initializers'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:48:in `each'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:48:in `tsort_each_child'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/initializable.rb:58:in `run_initializers'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/application.rb:353:in `initialize!'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/config/environment.rb:5:in `&lt;top (required)&gt;'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:292:in `require'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:292:in `block in require'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:258:in `load_dependency'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/activesupport-5.1.6/lib/active_support/dependencies.rb:292:in `require'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/application.rb:329:in `require_environment!'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/railties-5.1.6/lib/rails/application.rb:445:in `block in run_tasks_blocks'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:62:in `block (2 levels) in define'
  remote:        /tmp/build_3e8ae02f61a1fa36427570c9cf7d0b7d/vendor/bundle/ruby/2.4.0/gems/rake-12.3.1/exe/rake:27:in `&lt;top (required)&gt;'
  remote:        Tasks: TOP =&gt; environment
  remote:        (See full trace by running task with --trace)
  remote: 
  remote:  !
  remote:  !     Precompiling assets failed.
  remote:  !
  remote:  !     Push rejected, failed to compile Ruby app.
  remote: 
  remote:  !     Push failed
  remote: Verifying deploy...
  </code></pre>
  
  <p>I've been digging through many threads and have yet to find an answer, any help would be appreciated. Below is the file it's referencing. 
  /config/initializers/devise.rb</p>
  
  <pre><code>Devise.setup do |config|
    # The secret key used by Devise. Devise uses this key to generate
    # random tokens. Changing this key will render invalid all existing
    # confirmation, reset password and unlock tokens in the database.
    # Devise will use the `secret_key_base` as its `secret_key`
    # by default. You can change it below and use your own secret key.
    config.secret_key = '######'
  
  
  
    # Configure the class responsible to send e-mails.
    # config.mailer = 'Devise::Mailer'
  
    # Configure the parent class responsible to send e-mails.
    # config.parent_mailer = 'ActionMailer::Base'
  
    # ==&gt; ORM configuration
    # Load and configure the ORM. Supports :active_record (default) and
    # :mongoid (bson_ext recommended) by default. Other ORMs may be
    # available as additional gems.
    require 'devise/orm/active_record'
  
    # ==&gt; Configuration for any authentication mechanism
    # Configure which keys are used when authenticating a user. The default is
    # just :email. You can configure it to use [:username, :subdomain], so for
    # authenticating a user, both parameters are required. Remember that those
    # parameters are used only when authenticating and not when retrieving from
    # session. If you need permissions, you should implement that in a before filter.
    # You can also supply a hash where the value is a boolean determining whether
    # or not authentication should be aborted when the value is not present.
    # config.authentication_keys = [:email]
  
    # Configure parameters from the request object used for authentication. Each entry
    # given should be a request method and it will automatically be passed to the
    # find_for_authentication method and considered in your model lookup. For instance,
    # if you set :request_keys to [:subdomain], :subdomain will be used on authentication.
    # The same considerations mentioned for authentication_keys also apply to request_keys.
    # config.request_keys = []
  
    # Configure which authentication keys should be case-insensitive.
    # These keys will be downcased upon creating or modifying a user and when used
    # to authenticate or find a user. Default is :email.
    config.case_insensitive_keys = [:email]
  
    # Configure which authentication keys should have whitespace stripped.
    # These keys will have whitespace before and after removed upon creating or
    # modifying a user and when used to authenticate or find a user. Default is :email.
    config.strip_whitespace_keys = [:email]
  
    # Tell if authentication through request.params is enabled. True by default.
    # It can be set to an array that will enable params authentication only for the
    # given strategies, for example, `config.params_authenticatable = [:database]` will
    # enable it only for database (email + password) authentication.
    # config.params_authenticatable = true
  
    # Tell if authentication through HTTP Auth is enabled. False by default.
    # It can be set to an array that will enable http authentication only for the
    # given strategies, for example, `config.http_authenticatable = [:database]` will
    # enable it only for database authentication. The supported strategies are:
    # :database      = Support basic authentication with authentication key + password
    # config.http_authenticatable = false
  
    # If 401 status code should be returned for AJAX requests. True by default.
    # config.http_authenticatable_on_xhr = true
  
    # The realm used in Http Basic Authentication. 'Application' by default.
    # config.http_authentication_realm = 'Application'
  
    # It will change confirmation, password recovery and other workflows
    # to behave the same regardless if the e-mail provided was right or wrong.
    # Does not affect registerable.
    # config.paranoid = true
  
    # By default Devise will store the user in session. You can skip storage for
    # particular strategies by setting this option.
    # Notice that if you are skipping storage for all authentication paths, you
    # may want to disable generating routes to Devise's sessions controller by
    # passing skip: :sessions to `devise_for` in your config/routes.rb
    config.skip_session_storage = [:http_auth]
  
    # By default, Devise cleans up the CSRF token on authentication to
    # avoid CSRF token fixation attacks. This means that, when using AJAX
    # requests for sign in and sign up, you need to get a new CSRF token
    # from the server. You can disable this option at your own risk.
    # config.clean_up_csrf_token_on_authentication = true
  
    # When false, Devise will not attempt to reload routes on eager load.
    # This can reduce the time taken to boot the app but if your application
    # requires the Devise mappings to be loaded during boot time the application
    # won't boot properly.
    # config.reload_routes = true
  
    # ==&gt; Configuration for :database_authenticatable
    # For bcrypt, this is the cost for hashing the password and defaults to 11. If
    # using other algorithms, it sets how many times you want the password to be hashed.
    #
    # Limiting the stretches to just one in testing will increase the performance of
    # your test suite dramatically. However, it is STRONGLY RECOMMENDED to not use
    # a value less than 10 in other environments. Note that, for bcrypt (the default
    # algorithm), the cost increases exponentially with the number of stretches (e.g.
    # a value of 20 is already extremely slow: approx. 60 seconds for 1 calculation).
    config.stretches = Rails.env.test? ? 1 : 11
  
    # Set up a pepper to generate the hashed password.
    # config.pepper = '1ce8d56240e1fd617765ff43fab306c59332a9bc6fccc2cd84fe6665d27a116ea7299d13b9bc57c5d53a425066ceadf58f5405ac29fa86210208f7398af8024f'
  
    # Send a notification to the original email when the user's email is changed.
    # config.send_email_changed_notification = false
  
    # Send a notification email when the user's password is changed.
    # config.send_password_change_notification = false
  
    # ==&gt; Configuration for :confirmable
    # A period that the user is allowed to access the website even without
    # confirming their account. For instance, if set to 2.days, the user will be
    # able to access the website for two days without confirming their account,
    # access will be blocked just in the third day. Default is 0.days, meaning
    # the user cannot access the website without confirming their account.
    # config.allow_unconfirmed_access_for = 2.days
  
    # A period that the user is allowed to confirm their account before their
    # token becomes invalid. For example, if set to 3.days, the user can confirm
    # their account within 3 days after the mail was sent, but on the fourth day
    # their account can't be confirmed with the token any more.
    # Default is nil, meaning there is no restriction on how long a user can take
    # before confirming their account.
    # config.confirm_within = 3.days
  
    # If true, requires any email changes to be confirmed (exactly the same way as
    # initial account confirmation) to be applied. Requires additional unconfirmed_email
    # db field (see migrations). Until confirmed, new email is stored in
    # unconfirmed_email column, and copied to email column on successful confirmation.
    config.reconfirmable = true
  
    # Defines which key will be used when confirming an account
    # config.confirmation_keys = [:email]
  
    # ==&gt; Configuration for :rememberable
    # The time the user will be remembered without asking for credentials again.
    # config.remember_for = 2.weeks
  
    # Invalidates all the remember me tokens when the user signs out.
    config.expire_all_remember_me_on_sign_out = true
  
    # If true, extends the user's remember period when remembered via cookie.
    # config.extend_remember_period = false
  
    # Options to be passed to the created cookie. For instance, you can set
    # secure: true in order to force SSL only cookies.
    # config.rememberable_options = {}
  
    # ==&gt; Configuration for :validatable
    # Range for password length.
    config.password_length = 6..128
  
    # Email regex used to validate email formats. It simply asserts that
    # one (and only one) @ exists in the given string. This is mainly
    # to give user feedback and not to assert the e-mail validity.
    config.email_regexp = /\A[^@\s]+@[^@\s]+\z/
  
    # ==&gt; Configuration for :timeoutable
    # The time you want to timeout the user session without activity. After this
    # time the user will be asked for credentials again. Default is 30 minutes.
    # config.timeout_in = 30.minutes
  
    # ==&gt; Configuration for :lockable
    # Defines which strategy will be used to lock an account.
    # :failed_attempts = Locks an account after a number of failed attempts to sign in.
    # :none            = No lock strategy. You should handle locking by yourself.
    # config.lock_strategy = :failed_attempts
  
    # Defines which key will be used when locking and unlocking an account
    # config.unlock_keys = [:email]
  
    # Defines which strategy will be used to unlock an account.
    # :email = Sends an unlock link to the user email
    # :time  = Re-enables login after a certain amount of time (see :unlock_in below)
    # :both  = Enables both strategies
    # :none  = No unlock strategy. You should handle unlocking by yourself.
    # config.unlock_strategy = :both
  
    # Number of authentication tries before locking an account if lock_strategy
    # is failed attempts.
    # config.maximum_attempts = 20
  
    # Time interval to unlock the account if :time is enabled as unlock_strategy.
    # config.unlock_in = 1.hour
  
    # Warn on the last attempt before the account is locked.
    # config.last_attempt_warning = true
  
    # ==&gt; Configuration for :recoverable
    #
    # Defines which key will be used when recovering the password for an account
    # config.reset_password_keys = [:email]
  
    # Time interval you can reset your password with a reset password key.
    # Don't put a too small interval or your users won't have the time to
    # change their passwords.
    config.reset_password_within = 6.hours
  
    # When set to false, does not sign a user in automatically after their password is
    # reset. Defaults to true, so a user is signed in automatically after a reset.
    # config.sign_in_after_reset_password = true
  
    # ==&gt; Configuration for :encryptable
    # Allow you to use another hashing or encryption algorithm besides bcrypt (default).
    # You can use :sha1, :sha512 or algorithms from others authentication tools as
    # :clearance_sha1, :authlogic_sha512 (then you should set stretches above to 20
    # for default behavior) and :restful_authentication_sha1 (then you should set
    # stretches to 10, and copy REST_AUTH_SITE_KEY to pepper).
    #
    # Require the `devise-encryptable` gem when using anything other than bcrypt
    # config.encryptor = :sha512
  
    # ==&gt; Scopes configuration
    # Turn scoped views on. Before rendering "sessions/new", it will first check for
    # "users/sessions/new". It's turned off by default because it's slower if you
    # are using only default views.
    # config.scoped_views = false
  
    # Configure the default scope given to Warden. By default it's the first
    # devise role declared in your routes (usually :user).
    # config.default_scope = :user
  
    # Set this configuration to false if you want /users/sign_out to sign out
    # only the current scope. By default, Devise signs out all scopes.
    # config.sign_out_all_scopes = true
  
    # ==&gt; Navigation configuration
    # Lists the formats that should be treated as navigational. Formats like
    # :html, should redirect to the sign in page when the user does not have
    # access, but formats like :xml or :json, should return 401.
    #
    # If you have any extra navigational formats, like :iphone or :mobile, you
    # should add them to the navigational formats lists.
    #
    # The "*/*" below is required to match Internet Explorer requests.
    # config.navigational_formats = ['*/*', :html]
  
    # The default HTTP method used to sign out a resource. Default is :delete.
    config.sign_out_via = :delete
  
    # ==&gt; OmniAuth
    # Add a new OmniAuth provider. Check the wiki for more information on setting
    # up on your models and hooks.
    # config.omniauth :github, 'APP_ID', 'APP_SECRET', scope: 'user,public_repo'
  
    # ==&gt; Warden configuration
    # If you want to use other strategies, that are not supported by Devise, or
    # change the failure app, you can configure them inside the config.warden block.
    #
    # config.warden do |manager|
    #   manager.intercept_401 = false
    #   manager.default_strategies(scope: :user).unshift :some_external_strategy
    # end
  
    # ==&gt; Mountable engine configurations
    # When using Devise inside an engine, let's call it `MyEngine`, and this engine
    # is mountable, there are some extra configurations to be taken into account.
    # The following options are available, assuming the engine is mounted as:
    #
    #     mount MyEngine, at: '/my_engine'
    #
    # The router that invoked `devise_for`, in the example above, would be:
    # config.router_name = :my_engine
    #
    # When using OmniAuth, Devise cannot automatically set OmniAuth path,
    # so you need to do it manually. For the users scope, it would be:
    # config.omniauth_path_prefix = '/my_engine/users/auth'
  
    # ==&gt; Turbolinks configuration
    # If your app is using Turbolinks, Turbolinks::Controller needs to be included to make redirection work correctly:
    #
    # ActiveSupport.on_load(:devise_failure_app) do
    #   include Turbolinks::Controller
    # end
  end
  </code></pre>
  
  <p>database.yml (production part which seems to be working otherwise) </p>
  
  <pre><code>production:
    adapter: postgresql
    database: db/production.pg
  </code></pre>