AppendRows in Storage Write API Golang gets stuck in a loop

I am trying to send some data to BQ using Storage Write API. I use this example: https://cloud.google.com/bigquery/docs/write-api-batch

When my code is flat it works, but as soon as I put it into a loop it stucks forever at the second loop step and res, err := ms.AppendRows(ctx, pfProto):

    results := []*managedwriter.AppendResult{}
    for i := 0; i < len(skus); i = i + PFStocksLimit {
        lim := i + PFStocksLimit
        if lim > len(skus)-1 {
            lim = len(skus) - 1
        }
        stocks := getStocks(skus[i:lim])
        pfProto := prepProto(gd, stocks)

        res, err := ms.AppendRows(ctx, pfProto)
        if err != nil {
            log.Fatalf("AppendRows call error: %v", err)
        }
        results = append(results, res)
}

Funny enough when I remove results = append(results, res) it works. I do not understang what is wrong here.