Mapping one to many with Dapper

Trying to figure this out, but I can't get it to work. This query:

select MultiCollections.*, Collections.* from MultiCollections 
left join MultiCollectionCollections on MultiCollections.Id = MultiCollectionCollections.MultiCollectionId
left join  Collections on MultiCollectionCollections.CollectionId = Collections.Id
where MultiCollections.UserId=5

This will return this data:

As you can see, row 1 and 2 are from the same Title. The data behind them are books. Row 3 and 4 are also collections but don't have books.

I have two objects in my code: MultiCollection Collection

Both correspond with the data given in the result of the query: Id, UserId and Title are for object MultiCollection. Other data are for the object Collection.

I expect to see three MultiCollections in my C# code: Action Drama Fiction

Action will have 2 collections. Drama and Fiction should be empty.

Instead, I get 4 MultiCollections and none of them contains Collections. My C# code:

public IEnumerable<MultiCollection> GetAll(int userId)
    {
        string query = @"select MC.*, C.* from MultiCollections  MC
                        left join MultiCollectionCollections MCC on MC.Id = MCC.MultiCollectionId
                        left join  Collections C on MCC.CollectionId = C.Id
                        where UserId=" + userId;

        using (DbConnection connection = ConnectionFactory())
        {
            connection.Open();
            return connection.Query<MultiCollection, List<Collection>, MultiCollection>(query,
                (a, s) =>
                {
                    a.Collections = s;
                    return a;
                });
        }
    }

When running the code I would expect this:

Action    
    Collections    
       -> Book 1    
       -> Book 2     
Drama    
   Collections    
       Null     
Fiction    
    Collections    
        Null

I have no idea what I'm doing wrong.