Jekyll: "no implicit conversion of nil into String" Error occured when I install jekyll

 ~ gem install -n /usr/local/bin jekyll
ERROR:  While executing gem ... (TypeError)
    no implicit conversion of nil into String

What operating system are you using?

ProductName:    Mac OS X
ProductVersion: 10.11.4
BuildVersion:   15E65

Homebrew 0.9.9 (git revision 663f5; last commit 2016-04-24)
Homebrew/homebrew-core (git revision 8a15; last commit 2016-04-24) 

ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]

gem 2.5.1

What did you do?

(Please include the content causing the issue, any relevant configuration settings, and the command you ran)
only run command

gem install -n /usr/local/bin jekyll

What did you expect to see?

Install jekyll success