```ruby
require 'yaml'

module Rundock
  module Builder
    class TaskBuilder < Base
      DEFAULT_TASKS_FILE_PATH = './tasks.yml'

      def build(scenario_tasks)
        tasks = {} unless scenario_tasks

        return scenario_tasks unless @options[:tasks]
        if FileTest.exist?(@options[:tasks])
          tasks.merge!(YAML.load_file(@options[:tasks]).deep_symbolize_keys)
          Logger.info("merged tasks file #{@options[:tasks]}")
        elsif FileTest.exist?(DEFAULT_TASKS_FILE_PATH)
          Logger.warn("tasks file is not found. use #{DEFAULT_TASKS_FILE_PATH}")
          tasks.merge!(YAML.load_file(DEFAULT_TASKS_FILE_PATH).deep_symbolize_keys)
        else
          Logger.warn("Task path is not available. (#{@options[:tasks]})")
        end

        tasks
      end
    end
  end
end
```

Fix error if merging scenario task and external task
user
```ruby
require 'yaml'

module Rundock
  module Builder
    class TaskBuilder < Base
      DEFAULT_TASKS_FILE_PATH = './tasks.yml'

      def build(scenario_tasks)
        tasks = if scenario_tasks.nil?
                     {}
                   else
                     scenario_tasks
                   end

        return scenario_tasks unless @options[:tasks]
        if FileTest.exist?(@options[:tasks])
          tasks.merge!(YAML.load_file(@options[:tasks]).deep_symbolize_keys)
          Logger.info("merged tasks file #{@options[:tasks]}")
        elsif FileTest.exist?(DEFAULT_TASKS_FILE_PATH)
          Logger.warn("tasks file is not found. use #{DEFAULT_TASKS_FILE_PATH}")
          tasks.merge!(YAML.load_file(DEFAULT_TASKS_FILE_PATH).deep_symbolize_keys)
        else
          Logger.warn("Task path is not available. (#{@options[:tasks]})")
        end

        tasks
      end
    end
  end
end
```
assistant