To reproduce:
this.grid.Columns.Add(new GridViewDecimalColumn("col"));
for (int i = 0; i < 5; i++)
{
    this.grid.Rows.Add(i);
}

GridViewSummaryItem stDev = new GridViewSummaryItem()
{
    Name = "col",
    Aggregate = GridAggregateFunction.Var // or GridAggregateFunction.StDev
};
var row = new GridViewSummaryRowItem();
row.Add(stDev);
this.grid.SummaryRowsTop.Add(row);

This will throw an exception.