rails/rails · error · ArgumentError

helper must be a String, Symbol, or Module

Error message

helper must be a String, Symbol, or Module

What it means

Error "helper must be a String, Symbol, or Module" thrown in rails/rails.

Source

Thrown at actionpack/lib/abstract_controller/helpers.rb:43

      self._helpers = define_helpers_module(self)
    end

    def _helpers
      self.class._helpers
    end

    module Resolution # :nodoc:
      def modules_for_helpers(modules_or_helper_prefixes)
        modules_or_helper_prefixes.flatten.map! do |module_or_helper_prefix|
          case module_or_helper_prefix
          when Module
            module_or_helper_prefix
          when String, Symbol
            helper_prefix = module_or_helper_prefix.to_s
            helper_prefix = helper_prefix.camelize unless helper_prefix.start_with?(/[A-Z]/)
            "#{helper_prefix}Helper".constantize
          else
            raise ArgumentError, "helper must be a String, Symbol, or Module"
          end
        end
      end

      def all_helpers_from_path(path)
        helpers = Array(path).flat_map do |_path|
          names = Dir["#{_path}/**/*_helper.rb"].map { |file| file[_path.to_s.size + 1..-"_helper.rb".size - 1] }
          names.sort!
        end
        helpers.uniq!
        helpers
      end

      def helper_modules_from_paths(paths)
        modules_for_helpers(all_helpers_from_path(paths))
      end
    end

View on GitHub (pinned to 817fc2a147)

When it happens

Trigger: Thrown at actionpack/lib/abstract_controller/helpers.rb:43 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rails/rails@817fc2a147 (2026-08-04). Data as JSON: /data/errors/c3e58661f9bdaccf.json. Report an issue: GitHub.