Why the TweetVolume is null when I fetch the Top Trends?

Question

I'm fetching the top trends of twitter using tweetinvi, Some of the trends have the tweet_volume value null, Am I doing anything wrong? Why its null?

List<Tweet> topTrends = new List<Tweet>();

ITwitterCredentials creds = new TwitterCredentials("CONSUMER_KEY", "CONSUMER_SECRET", "ACCESS_TOKEN", "ACCESS_TOKEN_SECRET");
Auth.SetCredentials(creds);

var trends = Trends.GetTrendsAt(23424977);

for (int i = 0; i < trends.Trends.Count(); i++)
{
    Tweet tweet = new Tweet
    {
        Name = trends.Trends[i].Name,
        Volume = trends.Trends[i].TweetVolume,
        Url = trends.Trends[i].URL,
        Count = trends.Trends[i].PromotedContent
    };
    trendList.Add(tweet);
}