Modifying the example from the documentation by adding the field d with a Date value to the initial object, starting with a fresh database: db.orders.insert({ _id: ObjectId( "5085a95c8fada716c89d0021" ), ord_dt: ISODate( "2012-07-01T04:00:00Z" ), ship_dt: ISODate( "2012-07-02T04:00:00Z" ), item: { sku: "abc123" , price: 1.99, uom: "pcs" , qty: 25 } }); // Next command will cause an error db.orders.group({ key: { ord_dt: 1, 'item.sku' : 1 }, cond: { ord_dt: { $gt: new Date( '01/01/2012' ) } }, reduce: function ( curr, result ) { }, initial: {d: new Date()} }); Either removing the property in the finalize function or overwriting it avoids the issue: // Next command runs without error db.orders.group({ key: { ord_dt: 1, 'item.sku' : 1 }, cond: { ord_dt: { $gt: new Date( '01/01/2012' ) } }, reduce: function ( curr, result ) { }, initial: {d: new Date()}, finalize: function (result) { delete result.d; } }); // As does this one db.orders.group({ key: { ord_dt: 1, 'item.sku' : 1 }, cond: { ord_dt: { $gt: new Date( '01/01/2012' ) } }, reduce: function ( curr, result ) { result.d = new Date() }, initial: {d: new Date()} });