Custom Datagrid problem with a custom property

[scamp]

I am creating a custom datagrid that inherits from the
System.Web.UI.WebControls.DataGrid. I'm trying to add a property
called ScrollHeight which is of type Unit. Whenever I try to edit the
property from the Visual Studio designer I get the message "Invalid
property value - Not Implemented". Yet when I go to the HTML and enter
the attribute on the tag it works fine.
<cc1:MyDataGrid id="grid1" runat="server"
ScrollHeight="300px"></cc1:MyDataGrid>

What am I missing to get this to work from the Properties screen in the
designer?

Below is some code from the datagrid:

[ToolboxData("<{0}:MyDataGrid runat=server></{0}:ISDataGrid>")]
public class ISDataGrid : System.Web.UI.WebControls.DataGrid,
IPostBackDataHandler
{
[ Category("Layout") ]
public Unit ScrollHeight
{
get {
return (Unit) ViewState["ScrollHeight"];
}
set {
ViewState["ScrollHeight"] = value;
}
}
}