StreamReader is always EndOfStream=true even before reading

//var response = httpClient.GetAsync(url);

using (var fs = new FileStream("000000_0.deflate", FileMode.Open))
{
    //await response.Result.Content.CopyToAsync(fs);
    using (var deflate = new ZLibStream(fs, CompressionMode.Decompress, true))
    using (var reader = new StreamReader(deflate, Encoding.ASCII))
    {
        messages.Add($"EOS: {reader.EndOfStream}");

var url = $"http://url.to.download/{fileName}";
var response = httpClient.GetAsync(url);

using (var fs = new FileStream(_fileDir + fileName, FileMode.Create))
{
    await response.Result.Content.CopyToAsync(fs);
    using (var deflate = new ZLibStream(fs, CompressionMode.Decompress, true))
    using (var reader = new StreamReader(deflate, Encoding.ASCII))
    {
        messages.Add($"EOS: {reader.EndOfStream}");