Trying to extend the basket pipeline to allow removal of VAT if certain criteries are met.

What i want to do:

If a customer types in a VAT number i want to remove vat from order.

Have tried http://our.umbraco.org/projects/website-utilities/ucommerce/ucommerce-support/12407-Apply-Vat-on-cartaspx-page but think i have registred the pipeline wrong,

What i have so far:

ExcludeVAT.aspx

namespace WocaHelper
{
    public class MyPipelineTask : IPipelineTask<PurchaseOrder>
    {
        public PipelineExecutionResult Execute(PurchaseOrder subject)
        {
            bool ExcludeVAT = false;
            if (!bool.TryParse(subject["ExcludeVAT"], out ExcludeVAT)) return PipelineExecutionResult.Success;
            if (ExcludeVAT)
            {
                // Find the price group - assume only one configured
                //PriceGroup priceGroup = PriceGroup.All().FirstOrDefault(x => !x.Deleted);
                // Find that configured price group
                decimal vatRate = 0; //priceGroup.VATRate;
                foreach (var orderLine in subject.OrderLines)
                {
                    orderLine.VATRate = vatRate;
                    // VAT is stored on a per unit basis
                    orderLine.VAT = vatRate; // vatRate; //orderLine.Price * vatRate;
                    orderLine.Save();
                    // Line totals and VAT totals are calculated by a different task
                }
            }
            return PipelineExecutionResult.Success;
        }
    }
}

and in Basket.config

...
<value>WocaHelper.MyPipeLineTask</value>
<value>${Basket.CalculateVATTotal}</value>

<!-- Pipeline Tasks-->
        <component id="WocaHelper.MyPipelineTask"
                   service="UCommerce.Pipelines.IPipelineTask`1[[UCommerce.Entities.PurchaseOrder, UCommerce]], UCommerce"
                   type="WocaHelper.MyPipelineTask, WocaHelper"
                   lifestyle="Thread" />

in my basket usercontrol (onTextChange event on vatnumber textBox

var basket = SiteContext.Current.OrderContext.GetBasket();
            string vatnumber = txt_vat.Text;
            if (deliverToPaymentAddress1.Checked)
            {
                if (vatnumber != "" && ddl_country1.SelectedValue == "da-DK")
                {
                    UCommerce.Xslt.Library.SetOrderProperty("ExcludeVAT", "true");
                    Library.ExecuteBasketPipeline();
                }
            } 

With this setup i get error: 

Value cannot be null.