Instância não é criada

Segui o passo-a-passo do curso, mas não consegui com que uma instância fosse criada no EC2. Quando executo o comando vagrant up --provider=aws é exibida a seguinte informação:

An active machine was found with a different provider. Vagrant currently allows each machine to be brought up with only a single provider at a time. A future version will remove this limitation. Until then, please destroy the existing machine to up with a new provider.

Machine name: web

Active provider: virtualbox

Requested provider: aws

Meu Vagrantfile:

Vagrant.configure("2") do |config|
    #config.vm.box = "hashicorp/precise32"
    config.vm.box = "ubuntu_aws"
    config.vm.box_url = "https://github.com/mitchellh/vagrant-aws/raw/master/dummy.box"

    config.vm.provider :aws do |aws, override| 
        aws.access_key_id = "meu_access_key_id"
        aws.secret_access_key = "meu_secret_access_key"

        aws.keypair_name = "devops"
        aws.ami = "ami-01f05461" # Ubuntu Server 14.04 LTS
        aws.security_groups = ['devops']

        override.ssh.username = "ubuntu"
        override.ssh.private_key_path = "devops.pem"
    end

    config.vm.define :web do |web_config|
        web_config.vm.network "private_network", ip: "192.168.50.10"
        web_config.vm.provision "puppet" do |puppet|
            puppet.manifest_file = "web.pp"
        end

        web_config.vm.provider :aws do |aws|
            aws.tags = {'Name' => 'MusicJungle (vagrant)'}
        end
    end
end

Estou fazendo algo errado?