Flatten Nested List using AutoMapper

I'm trying to flatten a nested object into a DTO object in .NET 3.5. Most of what I've seen so far is to use AutoMapper to do this (using v1.x since I need to use .NET 3.5, unfortunately):

Here's what a snippet of my class structures look like:

public class RootObject
{
    [JsonProperty("BaseSupplier")]
    public BaseSupplier BaseSupplier { get; set; }

    [JsonProperty("BaseOrderShipmentLineitem")]
    public IList<BaseOrderShipmentLineitem> BaseOrderShipmentLineitem { get; set; }
}

public class BaseSupplier
{
    [JsonProperty("id")]
    public int Id { get; set; }

    [JsonProperty("name")]
    public string Name { get; set; }
}

public class BaseOrderShipmentLineitem
{
    [JsonProperty("id")]
    public int Id { get; set; }

    [JsonProperty("qty_delivered")]
    public int QtyDelivered { get; set; }

    [JsonProperty("BaseOrderLineitem")]
    public BaseOrderLineitem BaseOrderLineitem { get; set; }    
}

public class BaseOrderLineitem
{
    [JsonProperty("id")]
    public int Id { get; set; }

    [JsonProperty("product_sku")]
    public string ProductSku { get; set; }
}

public class ShipmentDetailsDTO
{
    public int BaseOrderShipmentLineitemId { get; set; }
    public string BaseSupplierName { get; set; }
    public string Sku { get; set; }
}

I've been trying something like this:

Mapper.CreateMap<BaseOrderLineitem, ShipmentDetailsDTO>()
    .ForMember(d => d.Sku, opts => opts.MapFrom(s => s.ProductSku));
Mapper.CreateMap<BaseOrderShipmentLineitem, ShipmentDetailsDTO>();
Mapper.CreateMap<RootObject, ShipmentDetailsDTO>()
    .ForMember(d => d.Sku, opts => opts.MapFrom(s => Mapper.Map<IEnumerable<BaseOrderLineitem>, IEnumerable<ShipmentDetailsDTO>>(s.BaseOrderShipmentLineitem.SelectMany(q => q.BaseOrderLineitem)).FirstOrDefault().Sku))
    ;

var model = Mapper.Map<IEnumerable<RootObject>, IEnumerable<ShipmentDetailsDTO>>(obj);

With that above code I'm getting an error on this bit s.BaseOrderShipmentLineitem.SelectMany(q => q.BaseOrderLineitem):

Cannot implicitly convert type 'IEnumerable<?>' to 'IEnumerable<BaseOrderLineitem>'. An explicit conversion exists (are you missing a cast?)

I'm not sure if it's something simple I'm just overlooking or not.