Consider this example, where Concrete represents a collection and happens to derive from Abstract, which is an abstract base class.

IQueryable<Concrete> concretes = collection.AsQueryable();
IQueryable<Abstract> abstracts = concretes;

var queryResults = abstracts.Where(...).ToList(); // results in MissingMethodException

System.MissingMethodException: Constructor on type 'MongoDB.Driver.Linq.Projector2[[[MongoDB.Bson.ObjectId, MongoDB.Bson, Version=1.5.0.26036, Culture=neutral, PublicKeyToken=f686731cfb9cc103]]' not found. at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) at MongoDB.Driver.Linq.SelectQuery.Execute() at MongoDB.Driver.Linq.MongoQueryable1.GetEnumerator()

Note – this used to work perfectly in Fluent mongo.