SyntaxError: Unexpected token h in JSON at position 0 at JSON.parse: mean stack

This is my ionic code

dialogflow(question) {
    let headers = new Headers();
    headers.append('Content-Type','application/json');
    return this.http.post('http://localhost:3000/api/dialogflow',question,{headers: headers})
    .map(res => res.json());
  }

In the console log I'm getting errors as

POST http://localhost:3000/api/dialogflow 400 (Bad Request) This is from routes files(back-end)

router.post('/dialogflow', function(req,res) {
  console.log('reached here');

In the back-end(mean) console it is throwing an error as

SyntaxError: Unexpected token h in JSON at position 0
    at JSON.parse (<anonymous>)
    at createStrictSyntaxError (D:\product\project-1\node_modules\body-parser\lib\types\json.js:158:10)
    at parse (D:\product\project-1\node_modules\body-parser\lib\types\json.js:83:15)
    at D:\product\project-1\node_modules\body-parser\lib\read.js:121:18
    at invokeCallback (D:\product\project-1\node_modules\raw-body\index.js:224:16)
    at done (D:\product\project-1\node_modules\raw-body\index.js:213:7)
    at IncomingMessage.onEnd (D:\product\project-1\node_modules\raw-body\index.js:273:7)
    at emitNone (events.js:106:13)
    at IncomingMessage.emit (events.js:208:7)
    at endReadableNT (_stream_readable.js:1064:12)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)

How can I resolve this?