操作符丢失 SQL语句

string cmdText = " select [MessageForSingle].*,S.Username as SenderName,R.Username as ReceiverName from [MessageForSingle] inner join [User] S on S.ID=MessageForSingle.Sender inner join [User] as R on R.ID=MessageForSingle.Receiver"               
  +" WHERE (MessageForSingle.Sender=@UserID AND MessageForSingle.Receiver=@UserID) OR (MessageForSingle.Sender=@FellowID AND MessageForSingle.Receiver=@UserID) ORDER BY CreateDate DESC";

结果提示操作符丢失    S.ID=MessageForSingle.Sender inner join [User] as R on R.ID=MessageForSingle.Receiver

什么问题