`UpperCaseHandler` および `RepeatHandler` 関数が `asl.MessageHandler` と互換性がないのはなぜですか?

cannot use UpperCaseHandler (type func(asl.MessageDelivery)) as type asl.MessageHandler in assignment

cannot use RepeatHandler (type func(asl.MessageDelivery)) as type asl.MessageHandler in argument to Repeater.ConsumeFunc

type MessageHandler func(MessageDelivery) (interface{}, error)

func UpperCaseHandler(md asl.MessageDelivery) (interface{}, error) {
    s.Reply(MessageTest{strings.ToUpper(md.Message.(string))}, md.Delivery)
    // Modified to return nil, nil as the MessageDelivery struct is handled by the ASL library
    return nil, nil
}

func RepeatHandler(md asl.MessageDelivery) (interface{}, error) {
    s.Reply(MessageTest{strings.Repeat(md.Message.(string), 5)}, md.Delivery)
    // Modified to return nil, nil as the MessageDelivery struct is handled by the ASL library
    return nil, nil
}