When you try to 3rd command : $ ./grader.js --checks checks.json --file index.html, error can occur.
then, check the node.js version.
[sourcecode] $ node --version[/sourcecode]
If the version of node is 0.6.x, you need update the version of node.js to table version.
[sourcecode language="bash"]<br />$sudo npm cache clean -f<br />$sudo npm install -g n<br />$sudo n stable<br />[/sourcecode]
(#ref:http://davidwalsh.name/upgrade-nodejs)
I resolved this error with updating v0.10.13
