DropDownList Data Binding To List <String> Not Working?

I'm trying to bind a List<String> to a DropDownList in a user control. I think I'm doing the right thing, but it seems that after my code executes the bindings are cleared. Here's the code for review!

User control:
<asp:DropDownList ID="subjectNameDropDown" runat="server"/>
<asp:DropDownList ID="yearLevelDropDown" runat="server"/>
Auto-generated designed code-behind:
public partial class NewSiteMetadataUserControl {
protected global::System.Web.UI.WebControls.DropDownList subjectNameDropDown;
protected global::System.Web.UI.WebControls.DropDownList yearLevelDropDown;
}
Code-behind:
public partial class NewSiteMetadataUserControl : UserControl
{
protected override void CreateChildControls()
{
subjectNameDropDown = new DropDownList();
yearLevelDropDown = new DropDownList();
}
protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
EnsureChildControls();
// Attempt 1
List<String> subjectNames = GetSubjectValues();
foreach (var subjectName in subjectNames)
subjectNameDropDown.Items.Add(subjectName);
subjectNameDropDown.DataBind();
// Attempt 2
List<String> yearLevels = GetYearLevelValues();
yearLevelDropDown.DataSource = yearLevels;
yearLevelDropDown.DataBind();
}
}

Should this approach work? If it should, how can I debug what happens after the code executes?