ruby/rubygems · error · ArgumentError

undefined class/module #{node.tag.sub("!ruby/object:", "")}

Error message

undefined class/module #{node.tag.sub("!ruby/object:", "")}

What it means

Error "undefined class/module #{node.tag.sub("!ruby/object:", "")}" thrown in ruby/rubygems.

Source

Thrown at lib/rubygems/yaml_serializer.rb:537

        validate_tag!(node.tag) if node.tag

        result = case node.tag
                 when "!ruby/object:Gem::Version"
                   build_version(node)
                 when "!ruby/object:Gem::Platform"
                   build_platform(node)
                 when "!ruby/object:Gem::Requirement", "!ruby/object:Gem::Version::Requirement"
                   build_requirement(node)
                 when "!ruby/object:Gem::Dependency"
                   build_dependency(node)
                 when nil
                   build_hash(node)
                 when "!ruby/object:Gem::Specification"
                   hash = build_hash(node)
                   hash[:tag] = node.tag
                   hash
                 else
                   raise ArgumentError, "undefined class/module #{node.tag.sub("!ruby/object:", "")}"
        end

        store_anchor(node.anchor, result)
      end

      def build_hash(node)
        result = {}
        node.pairs.each do |key_node, value_node|
          key = key_node.is_a?(Scalar) ? key_node.value.to_s : build_node(key_node).to_s
          value = build_node(value_node)

          if ARRAY_FIELDS.include?(key)
            value = normalize_array_field(value)
          end

          result[key] = value
        end
        result

View on GitHub (pinned to 86cbb817a3)

Solutions

1. The metadata references a class/module that is not defined; rebuild the gem with a current RubyGems

When it happens

Trigger: Thrown at lib/rubygems/yaml_serializer.rb:537 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of ruby/rubygems@86cbb817a3 (2026-08-23). Data as JSON: /api/errors/9308a4d41118c82b. Report an issue: GitHub.