You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: token.go
    path of file: ./repos/plumber/vendor/github.com/AthenZ/athenz/libs/go/zmssvctoken
    
    the code of the file until where you have to start completion: // Copyright 2016 Yahoo Inc.
// Licensed under the terms of t
    
func (n *NToken) assertValid() error {
	if n.Version == "" {
		return fmt.Errorf("Invalid token: missing version")
	}
	if n.Domain == "" {
		return fmt.Errorf("Invalid token: missing domain")
	}
	if n.Name == "" {
		return fmt.Errorf("Invalid token: missing name")
	}
	if n.KeyVersion == "" {
		return fmt.Errorf("Invalid token: missing key version")
	}
	if n.GenerationTime.IsZero() {
		return fmt.Errorf("Invalid token: missing generation time")
	}
	if n.ExpiryTime.IsZero() {
		return fmt.Errorf("Invalid token: missing expiry time")
	}
	return nil
}