Hi Linvi,
Long time no see :)

There's a bug in the code for fetching trends for a place when it has no results.
repro:

int woeid = 20094386;
IPlaceTrends trends = Trends.GetTrendsAt(woeid);

Will throw an exception.

I've fixed it at JoshKeegan@c244b1d

Sorry it's not a pull request, I haven't been using the up-to-date version of Tweetinvi for a while and haven't had time to make sure I can swap back from my custom version.
Cheers,
Josh