Error saving custom record: "The specified internal id is not allowed."

It's been awhile since I've needed to work with Web Services and am a bit rusty.

I'm doing a quick POC with WS and have run into an error that I'm not sure what to do with: "The specified internal id is not allowed."

Basically, I'm searching for a custom record, changing one of it's fields and trying to save it back.  Here is the code:

public bool FindDoctorsOrder(string orderID)  {      try      {          //  Create a new search object          CustomRecordSearchBasic crsb = new CustomRecordSearchBasic();            //  Tell the search what type of record to look for          crsb.recType = new RecordRef();          crsb.recType.internalId = "35";                    crsb.internalId                           = new SearchMultiSelectField();          crsb.internalId.searchValue               = new RecordRef[1];          crsb.internalId.searchValue[0]            = new RecordRef();          crsb.internalId.searchValue[0].internalId = orderID;          crsb.internalId.