C# - Persisting A Collection Backed By Viewstate In A CompositeControl?

Maybe it's been a long day but I'm having trouble persisting a collection backed by the ASP.NET ViewState in a CompositeControl. Here's a simplified version:

public class MyControl : CompositeControl
{
public Collection<MyObject> MyObjectCollection
{
get {
return (Collection<MyObject>)ViewState["coll"] == null ?
new Collection<MyObject>()
: (Collection<MyObject>)ViewState["coll"];
}
set { ViewState["coll"] = value; }
}
}
public partial class TestPage : System.Web.UI.Page
{
protected void btn_Click(object sender, EventArgs e)
{
myControl1.MyObjectCollection.Add(new MyObject());
}
}

When the button is clicked, the event hander btn_Click executes fine, but the setter for MyObjectCollection never gets called, hence the new MyObject() never gets persisted.