ASCII translit not working

My Go code:

``` Go
package main

import (
    "fmt"
    iconv "github.com/djimenez/iconv-go"
)

func main() {
    su := "aé"
    sa, err := iconv.ConvertString(su, "UTF-8", "ASCII//TRANSLIT")
    if err != nil {
        panic(err)
    }
    fmt.Println(sa)
}
```

Result:

```
a?
```

Expected result:

```
ae
```

Native iconv command:

```
echo "aé" | iconv -f "UTF-8" -t "ASCII//TRANSLIT"
```
