instructure/canvas-lms · error · ArgumentError

Invalid value for form: #

Error message

Invalid value for form: #{value}

What it means

HeadingsStartAtH2Rule#fix! only accepts values equal to the translated strings 'Change heading level to Heading 2' or 'Turn into a paragraph' and rewrites the element tag accordingly. Any other value raises ArgumentError 'Invalid value for form: ...' since the fixer cannot infer the intended transformation.

Solutions

1. Pass the exact I18n.t('Change heading level to Heading 2') or I18n.t('Turn into a paragraph') string as value, computed in the same locale as the request
2. Better: compare against stable keys instead of translated labels (refactor the rule to accept 'h2'/'p')
3. Ensure I18n.locale is consistent between rendering the fix form and processing fix!
4. Rescue ArgumentError and return a 400 with the accepted options

Example fix

// before
rule.fix!(elem, 'Change heading level to Heading 2') # fails in non-en locale
// after
rule.fix!(elem, I18n.t('Change heading level to Heading 2'))

// send stable keys, compare in the same locale
const accepted = [I18n.t('Change heading level to Heading 2'), I18n.t('Turn into a paragraph')];
if (!accepted.includes(value)) return alert('Invalid option');

begin
  rule.fix!(elem, value)
rescue ArgumentError => e
  render json: { error: e.message, accepted: ['h2','p'] }, status: :bad_request
end

When it happens

Trigger: Calling fix! with a value that doesn't byte-equal the current I18n.t translations, e.g. hard-coded English strings in a non-English locale, or a value key like 'h2' instead of the label; locale changed between form render and submission.

Common situations: Locale mismatch: page rendered in English, submitted under a different I18n.locale (per-user locale), so value compares false; tests passing raw English labels while specs run in another locale; API clients sending semantic values instead of labels.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Caption cannot be empty.
• Invalid scope value. Valid options are: #
• validation_error_filename
• validation_error_missing
• validation_error_too_long

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/84970f2994a04288. Report an issue: GitHub.

Appendix: source

Thrown at app/models/accessibility/rules/headings_start_at_h2_rule.rb:52

          label: I18n.t("How would you like to proceed?"),
          undo_text: I18n.t("Reformatted"),
          value: I18n.t("Change heading level to Heading 2"),
          action: I18n.t("Reformat"),
          options: [
            I18n.t("Change heading level to Heading 2"),
            I18n.t("Turn into a paragraph")
          ]
        )
      end

      def fix!(elem, value)
        case value
        when I18n.t("Change heading level to Heading 2")
          elem.name = "h2"
        when I18n.t("Turn into a paragraph")
          elem.name = "p"
        else
          raise ArgumentError, "Invalid value for form: #{value}"
        end
        { changed: elem }
      end

      def display_name
        I18n.t("Heading levels should start at level 2")
      end

      def message
        I18n.t(
          "This text is styled as a Heading 1, but there should only be one H1 on a web page — the page title. " \
          "Use Heading 2 or lower (H2, H3, etc.) for your content headings instead."
        )
      end

      def why
        [
          I18n.t(

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