vagrant up 時のエラーでハマった件

#現象
仕事で使っているPCの調子が悪かったので新しいPCにVM環境を構築していたら
エラーで「Vagrant up」出来ない状態に・・・。

バージョンはこんな感じ
VirtualBox 5.1 + Vagrant 1.8.4

#エラー内容

\Users\hoge>vagrant up

C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/util/platform.rb:85:in `block in windows_hyperv_admin?': undefined method `include?' for #<Vagrant::Util::Subprocess::Result:0x2de92e0> (NoMethodError)
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/util/platform.rb:89:in `call'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/util/platform.rb:89:in `windows_hyperv_admin?'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/plugins/providers/hyperv/provider.rb:20:in `usable?'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/environment.rb:381:in `block in default_provider'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/environment.rb:379:in `each'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/environment.rb:379:in `default_provider'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/plugin/v2/command.rb:174:in `block in with_target_vms'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/plugin/v2/command.rb:201:in `call'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/plugin/v2/command.rb:201:in `block in with_target_vms'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/plugin/v2/command.rb:183:in `each'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/plugin/v2/command.rb:183:in `with_target_vms'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/plugins/commands/up/command.rb:131:in `install_providers'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/plugins/commands/up/command.rb:85:in `execute'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/cli.rb:42:in `execute'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/lib/vagrant/environment.rb:302:in `cli'
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.8.4/bin/vagrant:174:in `<main>'```

#原因
7月12日にリリースされたばかりのVirtualBox5.1での不具合みたいです。
まだ、解決されていなさそう。


#対処法
サラッと調べてみても特に対処法が見当たらず。
素直にVirtualBox 5.0.24を入れなおしたら解決しました。
http://download.virtualbox.org/virtualbox/5.0.24/