The changes made here 9 month ago introduced a change whereby the collection of Field objects used to be an actual List<Field> that was returned and projected as an IList<Field> to what is now an Array.

The line below shows that .ToArray() was used when returning the collection of Field objects which doesn't allow modifying the data structure at runtime. We use dynamic properties on our Index schema so that when we call .Add(field) our runtime fails with a NotSupportedException: Collection was of a fixed size

I believe simply changing ToArray() to ToList() would solve this problem. Going to follow up with a PR.