Error while installing Sass on Win8.1: "ERROR: While executing gem (Errno::EACCES) Permission denied @ rb_sysopen"

I installed Ruby via the official installer, installed the non x64 version just in case. Then I tried to install sass via the command line like on the http://ift.tt/1f0fI2t page, The following ensued:

sgarcia C:\Users\sgarcia
> gem install sass
Fetching: sass-3.4.16.gem (100%)
ERROR:  While executing gem ... (Errno::EACCES)
    Permission denied @ rb_sysopen - C:/Program Files (x86)/Ruby22/bin/sass.bat

Any ideas what could be causing this?

via Chebli Mohamed