Why is my Netlify function throwing an error trying to JSON.parse() the event object?

I am trying to implement SendGrid through a Netlify serveless functions, however every time after submitting the form the function throws this error:

{
  "errorType": "SyntaxError",
  "errorMessage": "Unexpected token a in JSON at position 1",
  "trace": [
     "SyntaxError: Unexpected token a in JSON at position 1",
     "    at JSON.parse (<anonymous>)",
     "    at Runtime.exports.handler (/var/task/src/functions/sendmail.js:5:44)",
     "    at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)"
  ]
}

I have no idea what is wrong as from Netlify docs the event.body is a JSON string so why JSON.parse() is not working?

Github