go - Append slice to slice of slices

I have data structure:

type PosList []int
type InvertedIndex struct {
  Capacity  int
  Len       int
  IndexList []PosList
}

I have problem with Add method:

func (ii *InvertedIndex) Add(posList PosList, docId int) {
  if ii.Len == ii.Capacity {
    newIndexList := make([]PosList, ii.Len, (ii.Capacity+1)*2)
    for i := 0; i < ii.Len; i++ {
      newIndexList[i] = make([]int, len(ii.IndexList[i]))
      copy(newIndexList[i], ii.IndexList[i])
    }
    ii.IndexList = newIndexList
  }
  ii.IndexList = ii.IndexList[0 : ii.Len+2]
  ii.IndexList[docId] = posList
  return
}

Or, i try something like this:

func (ii *InvertedIndex) Add(posList PosList, docId int) {
  if ii.Len == ii.Capacity {
    newIndexList := make([]PosList, ii.Len, (ii.Capacity+1)*2)
    copy(newIndexList, ii.IndexList)
    ii.IndexList = newIndexList
  }
  ii.IndexList = ii.IndexList[0 : ii.Len+2]
  ii.IndexList[docId] = posList
  return
}

Both of them don't work, may be someone can explain how can i append a slice to structure like this.