repro'd!

gem-install-stacktrace-osx.log

$ rbx gem install zurb-foundation --backtrace
ERROR:  While executing gem ... (RuntimeError)
    can't modify frozen object
        kernel/common/marshal.rb:549:in `construct'
        kernel/common/marshal.rb:585:in `construct_array'
        kernel/common/integer.rb:194:in `times'
        kernel/common/marshal.rb:584:in `construct_array'
        kernel/common/marshal.rb:488:in `construct'
        kernel/common/marshal.rb:1176:in `load'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/specification.rb:1134:in `_load'
        kernel/common/marshal.rb:805:in `construct_user_defined'
        kernel/common/marshal.rb:498:in `construct'
        kernel/common/marshal.rb:1176:in `load'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/source.rb:108:in `fetch_spec'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/spec_fetcher.rb:144:in `spec_for_dependency'
        kernel/bootstrap/array.rb:66:in `each'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/spec_fetcher.rb:142:in `spec_for_dependency'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/dependency_installer.rb:220:in `find_gems_with_sources'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/dependency_installer.rb:272:in `find_spec_by_name_and_version'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/dependency_installer.rb:173:in `available_set_for'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/dependency_installer.rb:398:in `resolve_dependencies'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/dependency_installer.rb:351:in `install'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/commands/install_command.rb:180:in `install_gem'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/commands/install_command.rb:194:in `install_gems'
        kernel/bootstrap/array.rb:66:in `each'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/commands/install_command.rb:190:in `install_gems'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/commands/install_command.rb:146:in `execute'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/command.rb:305:in `invoke_with_build_args'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/command_manager.rb:167:in `process_args'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/command_manager.rb:137:in `run'
        /Users/amos/.rvm/rubies/rbx-head/lib/rubygems/gem_runner.rb:54:in `run'
        /Users/amos/.rvm/rubies/rbx-head/lib/bin/gem.rb:21:in `__script__'
        kernel/delta/codeloader.rb:66:in `load_script'
        kernel/delta/codeloader.rb:109:in `load_script'
        kernel/loader.rb:643:in `script'
        kernel/loader.rb:834:in `main'