Kindly assist to correct below codes

Question

i want to group after selecting

For Each row As LabtransType In svcClient.GetLabtrans(MdlConn.checkDB, " ").GroupBy(Function(x) x.result_ref).ToList()
TvWaiter.Nodes(0).Nodes.Add(New TreeNode(row.result_ref))
Next 

 i got this error

Additional information: Unable to cast object of type 'Grouping[System.String,Hospital_dbms.LabtransType]' to type 'Hospital_dbms.LabtransType'.

kindly any one with idea assist please