Error when overriding virtual method

Userlevel 4

I'm trying to override the following method in BOMCostRoll.cs:


protected virtual decimal? GetUnitCostFromINItemSite(INItemSite inItemSite)
return GetUnitCostFromINItemSiteTable(inItemSite);


Here is my override code:

using PX.Data;

using PX.Objects.IN;

namespace PX.Objects.AM


    //Use last cost instead of average or standard cost for cost rolls on BOM's.


    public abstract class GetUnitCostFromINItemCost_Ext : PXGraphExtension<BOMCostRoll>


        public delegate decimal? GetUnitCostFromINItemCostDelegate(INItemCost inItemCost);


        public virtual decimal? GetUnitCostFromINItemCost(INItemCost inItemCost, GetUnitCostFromINItemCostDelegate baseMethod)



                if (inItemCost != null &&  inItemCost.LastCost != null)



                    return inItemCost.LastCost;




            return null;




This is the error I get, no matter what I do:

No matter which method of overriding I’ve tried, I get an error “Value Cannot be Null, Parameter: con”


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