gem failed because of a type error. It seems only happens when gem source is Taobao no matter which gems specified or plugin required them.
ERROR: While executing gem ... (TypeError) no implicit conversion of nil into String I, [2015-06-04T06:21:40.571504 #37] INFO -- : gem install akismet -v 1.0.2 -i /var/www/discourse/plugins/discourse-akismet/gems/2.0.0 --no-document --ignore-dependencies
The problem is that gem can’t be downloaded. However I can manually install the gem by putting the same command which is weird to me.