This message should be fixed now, but I don't know how to reproduce this to test the problem. What page were you on before and what link/button did you click?
It looks like this code is triggered when adding something new to the cart from an 'update-quantities' form, but I'm not aware of any part of our site that does this rather than using the intended 'add-items' form. If possible I would like to update the form as well.