The parent id is missing. Help! SalesOrderDetail

Here's my code - it appears to work but when i try to create the salesorderdetail. it says "parent id is missing".

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Entity salesorderdetail = new Entity("salesorderdetail");

// Define the IDs needed for this sample.
private Guid _orderId;

var uom = "Default Unit";
//Query for the GUID of the UnitofMeasure using Name
QueryExpression queryUOM = new QueryExpression("uomschedule");
string[] cols = { "uomscheduleid", "name" };
queryUOM.Criteria = new FilterExpression();
queryUOM.Criteria.AddCondition("name", ConditionOperator.Equal, uom);
queryUOM.ColumnSet = new ColumnSet(cols);
var orderedBy = _service.RetrieveMultiple(queryUOM);
//Casting the reference to GUID
Guid uomscheduleGuid = (Guid)orderedBy[0].Id;

salesorderdetail["uomid"] = new EntityReference("uomschedule", new Guid(uomscheduleGuid.ToString()));
Console.WriteLine("UOM ID created");

salesorderdetail["quantity"] = Convert.ToDecimal(1);

var prodname = "NikeShoes";
//Query for the GUID of the Contact using fullname
QueryExpression queryProd = new QueryExpression("product");
string[] cols2 = { "productnumber", "name" };
queryProd.Criteria = new FilterExpression();
queryProd.Criteria.AddCondition("name", ConditionOperator.Equal, prodname);
queryProd.ColumnSet = new ColumnSet(cols2);
var prodby = _service.RetrieveMultiple(queryProd);
//Casting the reference to GUID
Guid prodGuid = (Guid)prodby[0].Id;

salesorderdetail["productid"] = new EntityReference("product", new Guid(prodGuid.ToString()));
Console.WriteLine("prodGuid created");

// Create an salesorder record
_orderId = _service.Create(salesorderdetail);

Console.Write("Sales Order Detail created, ");