怎么检查接口，老编译不过。求助

package main

import (
    "fmt"
)

type abc interface {
    up() bool
    down() bool
}

type myint uint32

func (m myint) up() bool {
    return true
}

func (m myint) down() bool {
    return false
}

func main() {
    var nn myint

    if ss, ok := nn.(abc); ok {
        fmt.Println("ok")
    }
}

老是提示 .\main.go:25: invalid type assertion: nn.(abc) (non-interface type myint on left)

哪位给看看？多谢