unable to copy struct with nested slices

[vishvanath-gojek]

Reproducible Example

type slice0 struct {
    sl1 []slice1
}

type slice1 struct {
    sl2 []slice2
}

type slice2 struct {
    sl2 []int
}

realdata := slice0{[]slice1{slice1{[]slice2{slice2{[]int{1, 2, 3}}}}}}
var copydata slice0
copier.Copy(&copydata, &realdata)
copydata.sl1[0] = slice1{[]slice2{slice2{[]int{10, 20, 30}}}}

Playground example https://play.golang.org/p/drVdfl5hx7M

Description

Running above example results in error index out of range.

[math-nao]

You are using unexported fields in your example. Copying to unexported field cannot be supported. You should use exported fields. See: https://play.golang.org/p/uIV1Sn8cU9I