install gh : undefined class/module Encoding

[juanpabloaj]

when I try install github gem

$ gem install json github                                                   
Building native extensions.  This could take a while...
Successfully installed json-1.5.3
Successfully installed github-0.6.2
2 gems installed
Installing ri documentation for json-1.5.3...
Installing ri documentation for github-0.6.2...
ERROR:  While executing gem ... (ArgumentError)
    undefined class/module Encoding