Shopify Dawn 2.0 Template: Adding Textarea to main-cart-items.liquid without cart error

Hello,
after following a couple of tutorials i was able to add an custom field to the cart.. which works, data is added to the order and the order goes through… but I recieve an error from the cart when it trys to update, does anyone know what could be causing this or a better way to insert a comment area into the cart section without errors.

Thankyou!

Daniel