Designer render error when using Composite Controls

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C#

ASP.NET

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:

<cc2:CustomGrid ID="CustomGrid1" runat="server" TotalItems="3"  />

Let me know if any more info is required.

Thanks in advance.