Sync Rule Parameter if reference object exists

Hi!

I am syncing user object to a SQL MA. In one field, i need the Managers employeenumber. So i achieved this with a sync rule Parameter as follows:

[//Target/Manager/employeenumber]

Now there are some user object that have no Manager. It is my understanding that i receive the following eventlog error for that reason:

System.ArgumentException: Cannot deference on non-instantiated Attribute Manager

Can someone give me a hint how i can check if a Manager is present and only then select the employeenumber?

I Vision the following:

IIF(ispresent(//Target/Manager),//Target/Manager/employeenumber,"")

Thanks for your time

Martin