I don't know C#, but what is happening is that your code is failing one of the tests (you should be able to read them in the Sample Tests panel in the bottom right of your screen).

Read the description part where it says:

If you get an empty array/list, return an empty array/list.

That's what you have to do, to pass the Sample Test with an empty input - you need to return whatever the C# equivalent of an empty array/list is.