Why I couldn't generate the Octopress blog

error_shell.sh

root@ypchen-ubuntu:~/octopress# rake generate
## Generating Site with Jekyll
unchanged sass/screen.scss
/usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:148:in `parse': couldn't parse YAML at line 20 column 0 (Psych::SyntaxError)
	from /usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:148:in `parse_stream'
	from /usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:119:in `parse'
	from /usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:106:in `load'
	from /usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/psych.rb:205:in `load_file'
	from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/jekyll-0.11.0/lib/jekyll.rb:119:in `configuration'
	from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/jekyll-0.11.0/bin/jekyll:207:in `<top (required)>'
	from /usr/local/rvm/gems/ruby-1.9.2-p290/bin/jekyll:19:in `load'
	from /usr/local/rvm/gems/ruby-1.9.2-p290/bin/jekyll:19:in `<main>'