“column”参数不能为空。参数名: column ？？？

DataColumn parentCol;
DataColumn childCol;
parentCol = objDataSet.Tables["message"].Columns["ID"];
childCol = objDataSet.Tables["reply"].Columns["messageID"];
DataRelation myRelation = new DataRelation("Relation",parentCol,childCol);
objDataSet.Relations.Add(myRelation);