Design-time render error for CompositeControl

[Fayu]

I have a composite control that works fine during runtime. However, during design time I receive an error: " '3' could not be set on property 'TotalItems'". When I first add this composite control on a page, it is rendered correctly. After I close that page and reopen it, I get this error. This is what my propery looks like:

   \[Browsable(true),
    Bindable(true)\]
    public long TotalItems
    {
        get
        {
            if (ViewState\["TotalItems"\] == null) return 10;

            return Convert.ToInt64(ViewState\["TotalItems"\]);
        }
        set
        {
            ViewState\["TotalItems"\] = value;

        }
    }

Here is what the class looks like:

\[Designer(typeof(CompositeControlDesigner)),
ParseChildren(true)\]
public sealed class CustomGrid: CompositeControl, INamingContainer, IEnumerator
{

...
}

Here is what the markup looks like:

Let me know if any more info is required. Thanks in advance.