can't convert String into Integer (TypeError)

/home/mini/ruby/lib/ruby/1.8/cgi.rb:940:in `[]'
/home/mini/ruby/lib/ruby/1.8/cgi.rb:940:in `user_agent'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:63:in `mobile_agent?'
/usr/local/httpd/htdocs/mini/diary/index.rb:66