Sign in with Microsoft
Sign in or create an account.
Select a different account.
You have multiple accounts
Choose the account you want to sign in with.


When you try to set up Microsoft Exchange Server 2019 Cumulative Update 10 (CU10) or Microsoft Exchange Server 2016 Cumulative Update 21 (CU21), Setup fails at the "PrepareAD" step and returns the following error message: 

System.NullReferenceException: Object reference not set to an instance of an object.


Setup fails if it cannot find or access the CN=Computers.<domain>.<com> container.


If the Computers container was renamed to some other name, use Active Directory Users and Computers to revert the container name to "Computers."

If the Computers container was deleted or you cannot rename the original Computers container, use the following steps to create a dummy Computers container to enable Setup to continue:

  1. Open ADSI Edit (Adsiedit.msc).

  2. Right-click ADSI Edit, and select Connect to.

    Screenshot that shows the steps to select "ADSI Edit" and "Connect to."

  3. In the Connection Settings window in the Name field, make sure that Default naming context is selected on the drop-down list, and then select OK.

    Screenshot that shows the Connection Settings dialog box.

  4. Expand Default naming context [<domain>], right-click DC=<domain>,DC=com, and then select New > Object.

  5. In the Create Object window in the class list, select container, and then select Next.

    Screenshot that shows selecting a class in the Create Object window.

  6. In the Value field, enter Computers, and then select Next.

    Screenshot that shows adding a value in the Create Object window.

  7. Select Finish.

    Screenshot that shows selecting "Finish" in the Create Object window.

  8. Force Active Directory (AD) replication.

  9. Resume or start Exchange Setup again.

Need more help?

Want more options?

Explore subscription benefits, browse training courses, learn how to secure your device, and more.

Communities help you ask and answer questions, give feedback, and hear from experts with rich knowledge.

Was this information helpful?

What affected your experience?
By pressing submit, your feedback will be used to improve Microsoft products and services. Your IT admin will be able to collect this data. Privacy Statement.

Thank you for your feedback!