Installation using proxy

hello,

I’m new to RonR. I tried installing rails using the --http-proxy option.

I typed this on the command line:
gem install rails --http-proxy username:passowrd@proxy-address

It retturned an error,

ERROR: While executing gem … (NoMethodError0
undefined method `proxy_open’ for #URI::Generic:0x2f45270

What did I miss?