Converting a user mailbox to a room/resource

Hello,

I have Exchange 2013.  I run the following command to convert a user to a room 

Set-Mailbox -Identity "Rom 888" -Type Room -EnableRoomMailboxAccount $true -RoomMailboxPassword (ConvertTo-SecureString -String "AnyPass1#3" -AsPlainText -Force)

You don't have permission to directly change a mailbox account password without providing old password. Reset Password
role is required for directly changing password.

I'm logged in to the exchange server as the domain admin - no modifications have been made to the domain admin account - it has the default rights.   If I open AD users and computers on that computer I can can change passwords.