此问题是由于在instrumenter.rb文件中插入了流浪(偶然)引起的.删除角色可以帮助我解决问题.我不确定为什么重新安装滑轨对我没有帮助.

This problem was due to a stray (and accidental) 's' being inserted in the instrumenter.rb file. Removing the character helped me fix the problem. I'm not sure about why reinstalling rails did not help me though.