SQL的Left join on的问题

[hjj1000]

我在写一个简单的SQL语句的时候遇到的一个问题

select P_ID,PQNum as RegNum,Project_Name as Name,Response_Sales as DSM,Project_Status as Status,CONVERT(varchar,Date_Create ,120) as RegistrationDate,CONVERT(varchar,LastUpdated ,120) as LastUpdate,customer_db1.customer_name from NormalP left join customer_db customer_db1 on customer_db1.Customer_ID= NormalP.GSH_AgentID

报得错是
服务器: 消息 8169，级别 16，状态 2，行 1
Syntax error converting from a character string to uniqueidentifier.

我觉得应该就算是表NormalP的字段GSH_AgentID为空也应该显示的啊，为什么就出错呢？