Spring MongoDB - Array objects come over null

I have a mongo collection that has an embedded array of ObjectId's, so the general format is this:

{
    "_id" : ObjectId("123abc..."),
    "Active" : [ 
        {
            "Id" : ObjectId("123abc...")
        }, 
        {
            "Id" : ObjectId("123abc...")
        }, 
        ...,
        {
            "Id" : ObjectId("123abc...")
        }
    ],
    "field1" : "blah blah",
    "field2" : "blah blah",
    "field3" : "blah blah",
    "field4" : "blah blah"
    "field5" : ObjectId("123abc...")
}

My Java class is pretty simple:

@Document(collection="MyCollection")
public class MyCollection {

    @Id
    private ObjectId id;
    private String field1;
    ...
    private ObjectId field5;
    @Field("Active")
    private List<Active> active;

    // Getters & Setters here

   public static class Active {
    @Field("Id")
    private ObjectId id;

    public ObjectId getId() {
        return id;
    }

    public void setId(ObjectId id) {
        this.id = id;
    }
  }
}

When I do the query, everything comes over correctly but the "Active" field. If there are 18 entries in that array, I will get a List of size 18, but the 18 entries of Active.class are all null - so it gets that it's a List, it just never pulls over the ObjectId values within the active array.

I'm using Spring and the Spring mongo driver.