A network engineer configures AAA on a router with the command 'aaa authentication login VTY-ACCESS group tacacs+ none' and applies it to the VTY lines. During a security audit, the auditor notes that when the TACACS+ servers were temporarily unreachable during maintenance, administrators were able to log in without providing any credentials. The engineer intended for local authentication to serve as the backup. Which change correctly resolves the security concern while preserving login access during a TACACS+ outage?