I'm getting this error

This exception was originally thrown at this call stack:
LokalPlanner.service.clsCalculateDiffMm.Name.get() in Summeries.cs
Infragistics.Windows.DataPresenter.SummaryDefinitionCollection.GetMatchingSummaries.AnonymousMethod__0(Infragistics.Windows.DataPresenter.SummaryDefinition)
Infragistics.Windows.DataPresenter.GridUtilities.Filter<T>(System.Collections.Generic.IEnumerable<T>, System.Predicate<T>)
Infragistics.Windows.DataPresenter.SummaryCalculatorSelectionControl.GetApplicableSummaryCalculators(Infragistics.Windows.DataPresenter.Field)
Infragistics.Windows.DataPresenter.SummaryCalculatorSelectionControl.UpdateSettings()
System.Windows.DependencyObject.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs)
System.Windows.FrameworkElement.OnPropertyChanged(System.Windows.DependencyPropertyChangedEventArgs)
System.Windows.DependencyObject.NotifyPropertyChange(System.Windows.DependencyPropertyChangedEventArgs)
System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex, System.Windows.DependencyProperty, System.Windows.PropertyMetadata, System.Windows.EffectiveValueEntry, ref System.Windows.EffectiveValueEntry, bool, bool, System.Windows.OperationType)
System.Windows.StyleHelper.ApplyTemplatedParentValue(System.Windows.DependencyObject, MS.Internal.FrameworkObject, int, ref MS.Utility.FrugalStructList<System.Windows.ChildRecord>, System.Windows.DependencyProperty, System.Windows.FrameworkElementFactory)
...
[Call Stack Truncated]

SummaryDefinition MMDiff = new SummaryDefinition();
MMDiff.SourceFieldName = "MMDiff";
MMDiff.Calculator = new clsCalculateDiffMm();
MMDiff.Key = "Field1.1";

MMDiff.StringFormat = "{0:0.00}";
grid.FieldLayouts[0].SummaryDefinitions.Add(MMDiff);

public class clsCalculateDiffMm : Infragistics.Windows.DataPresenter.SummaryCalculator
{
private int mmGammel = 0;
private int mmNy = 0;

public override string Description => throw new NotImplementedException();

public override string Name => throw new NotImplementedException();

public clsCalculateDiffMm()
{
}

public override void Aggregate(object dataValue, Infragistics.Windows.DataPresenter.SummaryResult summaryResult, Record record)
{
DataRecord dataRecord = (DataRecord)record;

mmGammel = int.Parse(dataRecord.Cells["MmÅrGammel"].Value.ToString());
mmNy = int.Parse(dataRecord.Cells["MmÅrNy"].Value.ToString());
}

public override void BeginCalculation(Infragistics.Windows.DataPresenter.SummaryResult summaryResult)
{
mmGammel = 0;
mmNy = 0;
}

public override bool CanProcessDataType(Type dataType)
{
throw new NotImplementedException();
}

public override object EndCalculation(Infragistics.Windows.DataPresenter.SummaryResult summaryResult)
{
double diff;
diff = mmNy - mmGammel;
return diff;
}
}

<igWPF:NumericField Label="Mm År Gammel" Name="MmÅrGammel" ></igWPF:NumericField>
<igWPF:NumericField Label="% Mm Diff" Name="PrmMMDiff" ></igWPF:NumericField>