Invalid Column Name Error in Script Task

I'm running into some trouble when trying to populate a lookup column from a script task.  I invariably get the error "System.Data.SqlClient.SqlException (0x80131904): Invalid column name 'UsrVMProductCode'."

This is the column in question:

And here is the relevant part of the script task:

var esq = new EntitySchemaQuery(UserConnection.EntitySchemaManager, "UsrWaterplayVMProductCode");
 
var ColId = esq.AddColumn("Id");
var ColName = esq.AddColumn("Name");
 
var entities = esq.GetEntityCollection(UserConnection);
var productCodes = new Dictionary<string,string>();
foreach (var entity in entities) {
	productCodes[entity.GetColumnValue(ColName.Name).ToString()] = entity.GetColumnValue(ColId.Name).ToString();
}
 
foreach (var pd in resultObj.UpdatedParts) {
	if (pd.PartId != "") {
		var code = "";
		if (pd.VMProductCode != "") {
			code = productCodes[pd.VMProductCode];
		}
		var update = new Update(UserConnection, "Product")
			.Set("Name", Column.Parameter(pd.Description))
			.Set("Price", Column.Parameter(pd.Price))
			.Set("UsrProductionCost", Column.Parameter(pd.Cost))
			.Set("IsArchive", Column.Parameter(pd.Inactive));
		if (code != "") {
			update.Set("UsrVMProductCode", Column.Parameter(code));
		}
		update.Where("Code").IsEqual(Column.Parameter(pd.PartId));
		if (update.Execute() > 0) {
			resultUpdate += "Product " + pd.PartId + " updated.\\r\\n";
		}
	}
}

I have confirmed through debug logging that the code variable contains a GUID.  The script also worked fine before I tried to set UsrVMProductCode, while attempting to set another lookup column in this way (Currency) gave a similar error.  Is there something special I need to do to set a lookup value with the Insert/Update objects?