NoMethodError (undefined method `downcase' for nil:NilClass)のエラー解消

前提

rialsでユーザー認証（devise, devise_token_auth）のapi疎通を確認するためにcurlコマンドを使用して確認していたのですがapiと関係ない部分でエラーが発生しており、エラーメッセージを検索して調べてみていますが、解決できないため質問させていただきます。

実現したいこと

エラーの解決

発生している問題・エラーメッセージ

エラーメッセージを読むとkがnilのことがあるために発生している問題かと思うのですが、このkが何を表しているか、どこで使用されているか見つけられずに詰まっています。

NoMethodError (undefined method `downcase' for nil:NilClass

        canonical = k.downcase.freeze
                     ^^^^^^^^^):
  
rack (2.2.4) lib/rack/utils.rb:464:in `[]='
rack (2.2.4) lib/rack/utils.rb:432:in `block in initialize'
rack (2.2.4) lib/rack/utils.rb:432:in `each'
rack (2.2.4) lib/rack/utils.rb:432:in `initialize'
rack (2.2.4) lib/rack/utils.rb:425:in `new'
rack (2.2.4) lib/rack/utils.rb:425:in `[]'
rack (2.2.4) lib/rack/etag.rb:29:in `call'
rack (2.2.4) lib/rack/conditional_get.rb:40:in `call'
rack (2.2.4) lib/rack/head.rb:12:in `call'
activerecord (7.0.3.1) lib/active_record/migration.rb:603:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/callbacks.rb:27:in `block in call'
activesupport (7.0.3.1) lib/active_support/callbacks.rb:99:in `run_callbacks'
actionpack (7.0.3.1) lib/action_dispatch/middleware/callbacks.rb:26:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/actionable_exceptions.rb:17:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/debug_exceptions.rb:28:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/show_exceptions.rb:26:in `call'
railties (7.0.3.1) lib/rails/rack/logger.rb:40:in `call_app'
railties (7.0.3.1) lib/rails/rack/logger.rb:25:in `block in call'
activesupport (7.0.3.1) lib/active_support/tagged_logging.rb:114:in `block in tagged'
activesupport (7.0.3.1) lib/active_support/tagged_logging.rb:38:in `tagged'
activesupport (7.0.3.1) lib/active_support/tagged_logging.rb:114:in `tagged'
railties (7.0.3.1) lib/rails/rack/logger.rb:25:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/remote_ip.rb:93:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/request_id.rb:26:in `call'
rack (2.2.4) lib/rack/runtime.rb:22:in `call'
activesupport (7.0.3.1) lib/active_support/cache/strategy/local_cache_middleware.rb:29:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/server_timing.rb:20:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/executor.rb:14:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/static.rb:23:in `call'
rack (2.2.4) lib/rack/sendfile.rb:110:in `call'
actionpack (7.0.3.1) lib/action_dispatch/middleware/host_authorization.rb:137:in `call'
rack-cors (1.1.1) lib/rack/cors.rb:100:in `call'
railties (7.0.3.1) lib/rails/engine.rb:530:in `call'
puma (5.6.4) lib/puma/configuration.rb:252:in `call'
puma (5.6.4) lib/puma/request.rb:77:in `block in handle_request'
puma (5.6.4) lib/puma/thread_pool.rb:340:in `with_force_shutdown'
puma (5.6.4) lib/puma/request.rb:76:in `handle_request'
puma (5.6.4) lib/puma/server.rb:441:in `process_client'
puma (5.6.4) lib/puma/thread_pool.rb:147:in `block in spawn_thread'

該当のソースコード

// gemfile
source "https://rubygems.org"
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby "3.1.2"

# Bundle edge Rails instead: gem "rails", github: "rails/rails", branch: "main"
gem "rails", "~> 7.0.3", ">= 7.0.3.1"

# Use postgresql as the database for Active Record
gem "pg", "~> 1.1"

# Use the Puma web server [https://github.com/puma/puma]
gem "puma", "~> 5.0"

# Build JSON APIs with ease [https://github.com/rails/jbuilder]
# gem "jbuilder"

# Use Redis adapter to run Action Cable in production
# gem "redis", "~> 4.0"

# Use Kredis to get higher-level data types in Redis [https://github.com/rails/kredis]
# gem "kredis"

# Use Active Model has_secure_password [https://guides.rubyonrails.org/active_model_basics.html#securepassword]
# gem "bcrypt", "~> 3.1.7"

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem "tzinfo-data", platforms: %i[ mingw mswin x64_mingw jruby ]

# Reduces boot times through caching; required in config/boot.rb
gem "bootsnap", require: false

# Use Active Storage variants [https://guides.rubyonrails.org/active_storage_overview.html#transforming-images]
# gem "image_processing", "~> 1.2"

# Use Rack CORS for handling Cross-Origin Resource Sharing (CORS), making cross-origin AJAX possible
gem "rack-cors"

# 認証用
gem 'devise'
gem 'devise_token_auth', '>= 1.2.0', git: "https://github.com/lynndylanhurley/devise_token_auth"
gem 'devise-i18n'

group :development, :test do
  # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem
  gem "debug", platforms: %i[ mri mingw x64_mingw ]
end

group :development do
  # Speed up commands on slow machines / big apps [https://github.com/rails/spring]
  # gem "spring"
end