trying to add product in cart through error in console

[azadpunchy]

Steps to reproduce:

1. visit https://bookstore-khoado.herokuapp.com/
2. sigin in as existing user
3. add any product to cart

Actual Result:
blank page appear and through error in console 'SyntaxError: Unexpected token u in JSON at position 0'

[azadpunchy]

reported mistakenly