Golang，Go：隐式调用接口函数？

Question

http://play.golang.org/p/xjs-jwMsr7

I have this function

 func (e *MyError) Error() string {
    return fmt.Sprintf("AT %v, %s", e.When, e.What)
 } 

But

as you see below, I never called it but how come it is called in the final output?

type MyError struct {
    When time.Time
    What string
}

func (e *MyError) Error() string {
    return fmt.Sprintf("AT %v, %s", e.When, e.What)
}

func run() error {
    return &MyError{
        time.Now(), "it didn't work",
    }
}

func main() {
    if err := run(); err != nil {
        fmt.Println(err)
    }
}