Using \U or \u in a prompt with the mysql command-line client could
  crash when connecting to the instance manager, since it does not return
  information about the user when asked by the client. This is fixed by
  having the client use what it knowns about the user (or giving up and
  saying "(unknown)").


client/mysql.cc:
  init_username() may not retrieve a username (such as from the instance
  manager), in which case we fall back to what was specified on the command
  line (or .cnf file) or finally (unknown).