Problem in Using! about how2 HOT 1 CLOSED

if(link.indexOf('stackoverflow.com') !== -1) {
            ^

TypeError: Cannot read property 'indexOf' of null
    at parseStackoverflowQuestionId (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/lib/utils.js:32:13)
    at isValidGoogleLink (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/lib/utils.js:56:12)
    at Array.filter (<anonymous>)
    at /Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/lib/how2.js:95:10
    at Request._callback (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/node_modules/google/lib/google.js:85:7)
    at Request.self.callback (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/node_modules/request/request.js:186:22)
    at Request.emit (events.js:180:13)
    at Request.<anonymous> (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/node_modules/request/request.js:1163:10)
    at Request.emit (events.js:180:13)
    at IncomingMessage.<anonymous> (/Users/jamesalbert/.nodenv/versions/9.10.1/lib/node_modules/how-2/node_modules/request/request.js:1085:12)