SeleniumHQ/selenium · error · TypeError

expected one of #{VALID_PREFERENCE_TYPES.inspect}, got #{val

Error message

expected one of #{VALID_PREFERENCE_TYPES.inspect}, got #{value.inspect}:#{value.class}

What it means

Raised by Firefox::Profile#[]= (TypeError) when the preference value is not one of VALID_PREFERENCE_TYPES ([TrueClass, FalseClass, Integer, Float, String]). Firefox about:config preferences only accept boolean, numeric, or string scalar values, so the setter rejects anything else (arrays, hashes, symbols, nil).

Source

Thrown at rb/lib/selenium/webdriver/firefox/profile.rb:101

          install_extensions(profile_dir)
          delete_lock_files(profile_dir)
          delete_extensions_cache(profile_dir)
          update_user_prefs_in(profile_dir)

          profile_dir
        end

        #
        # Set a preference for this particular profile.
        #
        # @see http://kb.mozillazine.org/About:config_entries
        # @see http://preferential.mozdev.org/preferences.html
        #

        def []=(key, value)
          unless VALID_PREFERENCE_TYPES.any? { |e| value.is_a? e }
            raise TypeError, "expected one of #{VALID_PREFERENCE_TYPES.inspect}, got #{value.inspect}:#{value.class}"
          end

          if value.is_a?(String) && Util.stringified?(value)
            raise ArgumentError, "preference values must be plain strings: #{key.inspect} => #{value.inspect}"
          end

          @additional_prefs[key.to_s] = value
        end

        def port=(port)
          WebDriver.logger.deprecate('Firefox::Profile#port=', 'the Service class', id: :firefox_profile)
          self[WEBDRIVER_PREFS[:port]] = port
        end

        def secure_ssl=(value)
          WebDriver.logger.deprecate('Firefox::Profile#secure_ssl=', id: :firefox_profile)
          @secure_ssl = value
        end

View on GitHub (pinned to aa36b38e69)

Solutions

1. Convert the value to one of the allowed types before assigning (symbol->string via to_s, etc.).
2. Use true/false for booleans, integers/floats for numbers, and plain strings for text.
3. If you need to remove a preference, manage the underlying prefs hash directly rather than assigning nil.

Example fix

// before
profile['network.proxy.type'] = :auto  # Symbol -> TypeError

// after
profile['network.proxy.type'] = 'auto'  # plain string
# or for known numeric codes
profile['network.proxy.type'] = 4

ALLOWED = [TrueClass, FalseClass, Integer, Float, String]
value = coerce(value) unless ALLOWED.any? { |t| value.is_a?(t) }
profile[key] = value

def valid_pref_value?(value)
  [TrueClass, FalseClass, Integer, Float, String].any? { |t| value.is_a?(t) }
end

When it happens

Trigger: Calling profile[key] = value with a Symbol (e.g. :none), an Array, a Hash, nil, or any non-scalar; passing a numeric as a string when a real number was intended is fine, but a Symbol like :auto will fail.

Common situations: Copying preference values from documentation that use symbols; deserializing JSON config into nested structures and assigning a Hash; passing nil to 'unset' a preference (not supported).

Related errors

• preference values must be plain strings: #{key.inspect} => #
• expected #{Proxy.name}, got #{proxy.inspect}:#{proxy.class}
• Port needs to be an integer
• Port number must be in the range 1..65535
• Please pass in a Boolean to this call

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/1d8566954df42b09. Report an issue: GitHub.