After notice of an assignment the debtor is liable to the assignee.1 Subsequent payment to the assignor,2 or to subsequent attaching creditors,3 or a subsequent contract with the assignor,4 or a settlement with him,5 or release from him,6 will not protect the debtor as against the assignee. So the debtor cannot set off against the assigned debt subsequent advances made by him to the assignor.7 But if the claim assigned is invalid the debtor is not liable to the assignee for a payment made to the assignor to avoid litigation.8 However, if notice of assignment is given to a debtor after he has given his check to the assignor, he is not bound to stop the check in order to protect himself from liability to the assignee.9