Web Forms :: Assignment Of FormView Control In Design-time To User Defined Composite Control

i create a composite control as can be seen in code below, and add this control to webform, assign FormView1 to the HeaderControlName property, Run the page, the system will generate parse error message :
Cannot create an object of type 'System.Web.UI.WebControls.FormView' from its string representation 'FormView1' for the 'HeaderControlName' property. However, if i don't assign FormView1 in design screen and manually (through coding) assign FormView1 to the custom control on init, it works as expected.

custom control
public class CmdTest : CompositeControl
{
public virtual FormView HeaderControlName
{
get
{
object oObject = ViewState["HeaderControlName"];
return (oObject == null) ? null : (FormView)oObject;
}
set
{
ViewState["HeaderControlName"] = value;
}
}
protected override void CreateChildControls(
{
Controls.Clear();
Button xx = new Button();
Controls.Add(xx);
}
}
webpage.aspx (assign FormView1 in design time, it will generate error)
<Utils:CmdTest ID="CmdTest1" runat="server" HeaderControlName="FormView1" />
webpage.aspx (didn't assign FormView1 in ASPX, but assign it on Init Code, it works)
<Utils:CmdTest ID="CmdTest1" runat="server" oninit="CmdTest1_Init" />
Webpage.aspx.cs
protected void BsCmdTest1_Init(object sender, EventArgs e)
{
CmdTest1.HeaderControlName = FormView1;
}