An agent message is an attempted action upon the information state of the receiver that, if successful, would cause the receiver to move to a new information state. A model of normative communication can define when messages are not merely unsuccessful but instead are illegal or impossible actions upon the receiver's internal state. The model uses the preconditions of the other core message types, coupled with a model of task interdependencies, agent roles, and belief-desire-intention elements, to define the preconditions for sending a canonical not-understood error message. By defining the space of messages that are legal actions on an agent's internal state, a normative communication model also defines a set of 'reasons' that can accompany the error message. A not-understood error message signals a mismatch between agent interaction models and the accompanying reason opens the possibility for agents to realign their respective models. The paper discusses the matters arising from this possibility. This approach assumes that normative communication behavior reflects normative domain behavior. It also assumes that each agent accesses the normative model, in contrast with more centralized frameworks for defining normative interaction among agents and identifying interaction errors.