怎么给基础资料字段赋值？

我做的插件，满足条件清空字段，我赋值为空，不好使，帮我看看
[code] public override void DataChanged(DataChangedEventArgs e)
{
base.DataChanged(e);
string a;

if ((a = e.Field.Key.ToUpperInvariant()) != null)
{
if (a == "FCUSTID")
{
var s = e.NewValue;

string SQL = @"select * from T_BD_CUSTOMER where F_ZWX_DATE string sWhere = string.Empty;
sWhere = string.Format(SQL, s);

object b = DBUtils.Execute(this.Context, sWhere);
if (b != null)
{
this.Model.SetValue("CustId", null);
this.View.ShowMessage("该客户存在证照过期情况，不允许选择", MessageBoxType.Notice);
}
}
}
}[/code]this.Model.SetValue("CustId", null);
我用CustId，FCustId，FCUSTID 都试了 不能赋值