MongoDB Unable to determine the serialization information

I'm having a problem with Mongodb querying. Here's the classes:

    class A
    { 

    }
    class B : A
    { 

    }
    class C : A
    {
        public string prop1 { get; set; }
    }
    class D
    {
        public List<A> collection1 { get; set; }
    }

And the query:

   var query1 = Query<D>.ElemMatch(d => d.collection1.OfType<C>(), 
             builder => builder.EQ(c => c.prop1, "val1"));

In a nutshell I want to select all instances of class D whose collection "collection1" contains at least one instance of class C whose property "prop1" equals "val1".

This keeps throwing "Unable to determine the serialization information for the expression" out of the blue.

What am I doing wrong? I've tried decorating "A" with BsonKnownTypes(typeof(B), typeof(C)) to no avail.

PS: if I had only one class A:

    class A
    { 
        public string prop1 { get; set; }
    }
    class D
    {
        public List<A> collection1 { get; set; }
    }

then this code works and does exactly what I want:

   var query1 = Query<D>.ElemMatch(d => d.collection1, 
             builder => builder.EQ(c => c.prop1, "val1"));

but it's not what I want, I want to keep "B" and "C" that inherit from "A".

Copyright Notice：Content Author:「user3664916」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/26216185/mongodb-unable-to-determine-the-serialization-information