The following throws an exception in BsonClassMap, in one of the internal checks:

public class SomeBaseType
{
public int X

{get; set;}
}

public class MyType : SomeBaseType
{
public int Y {get; set;}

}

_collection.AsQueryable().Select(collType => new MyType

{ Y = collType.SomeField; }

);

Basically, if the projection expression attempts to populate a member not directly in the type being constructed.

I will update this issue with more complete code and if possible a PR - just wanted to log it down before I forget.!