github/markup · error · ArgumentError

Can not render a nil.

Error message

Can not render a nil.

What it means

GitHub::Markup.render_s raises ArgumentError 'Can not render a nil.' when the content argument is nil. render_s is the symbol-keyed entry point (e.g. render_s(:markdown, text)), and the guard exists because the underlying renderer gems expect a String; passing nil through would surface as opaque NoMethodError/TypeError failures deep inside the renderer. The check is a fail-fast contract on the public API.

Source

Thrown at lib/github/markup.rb:51

    def markup_impls
      markups.values
    end

    def preload!
      markup_impls.each(&:load)
    end

    def render(filename, content, symlink: false, options: {})
      if (impl = renderer(filename, content, symlink: symlink))
        impl.render(filename, content, options: options)
      else
        content
      end
    end

    def render_s(symbol, content, options: {})
      raise ArgumentError, 'Can not render a nil.' if content.nil?

      if markups.key?(symbol)
        markups[symbol].render(nil, content, options: options)
      else
        content
      end
    end

    def markup(symbol, gem_name, regexp, languages, opts = {}, &block)
      impl = GemImplementation.new(regexp, languages, gem_name, &block)
      markup_impl(symbol, impl)
    end

    def markup_impl(symbol, impl)
      if markups.key?(symbol)
        raise ArgumentError, "The '#{symbol}' symbol is already defined."
      end
      markups[symbol] = impl

View on GitHub (pinned to 76e2682193)

Solutions

1. Pass an empty string instead of nil when the source may be absent: render_s(:markdown, content || '').
2. Guard at the boundary: skip rendering entirely when content.nil? and handle the empty case in the caller.
3. If nil means 'bug', keep the exception but add context by rescuing ArgumentError and re-raising with the symbol/filename you were rendering.

Example fix

# before
GitHub::Markup.render_s(:markdown, params[:body])
# params[:body] is nil => ArgumentError: Can not render a nil.

# after
GitHub::Markup.render_s(:markdown, params[:body].to_s)

def safe_render_s(symbol, content, options: {})
  return '' if content.nil?            # or raise your own typed error
  raise ArgumentError, 'content must be a String' unless content.is_a?(String)
  GitHub::Markup.render_s(symbol, content, options: options)
end

# Narrow before calling:
def renderable_content?(content)
  content.is_a?(String) # render_s only accepts String; nil raises 'Can not render a nil.'
end

begin
  GitHub::Markup.render_s(:markdown, content)
rescue ArgumentError => e
  raise unless e.message == 'Can not render a nil.'
  '' # deliberate empty output for absent bodies
end

When it happens

Trigger: Calling GitHub::Markup.render_s(:markdown, nil), render_s(:rst, nil), etc.; passing a variable that came back nil from File.read on a missing file, a nil database column, a nil params value, or a test fixture that was never set.

Common situations: Optional body/description fields that are nil for some records; controller code doing render_s(:markdown, params[:body]) without presence checks; specs with let-it-be blocks defaulting to nil; reading files whose path was computed incorrectly so read returned nil via rescue.

Related errors

• stderr
• The '#{symbol}' symbol is already defined.
• subclasses of GitHub::Markup::Implementation must define #re
• unknown commonmarker option: #{opt.inspect}
• unknown commonmarker extension: #{ext.inspect}

AI-assisted analysis of github/markup@76e2682193 (2026-08-21). Data as JSON: /api/errors/568e2495edfc9db2. Report an issue: GitHub.