windows 7 中安装DockerToolBox总是报错

Machine default already exists in VirtualBox.
Starting machine default...
Started machines may have new IP addresses. You may need to re-run the docker-machine env command.
Setting environment variables for machine default...
Unexpected error getting machine url: exit status 255

                    ##         .
              ## ## ##        ==
           ## ## ## ## ##    ===
       /"""""""""""""""""\___/ ===
  ~~~ {~~ ~~~~ ~~~ ~~~~ ~~~ ~ /  ===- ~~~
       \______ o           __/
         \    \         __/
          \____\_______/

exit status 255
docker is configured to use the default machine with IP
For help getting started, check out the docs at https://docs.docker.com
求解答？

http://blog.csdn.net/zistxym/article/details/42918339