Access denied when trying to create IIS backup

I'm trying create a backup by running:

%windir%\system32\inetsrv\appcmd.exe add backup "MyBackupName"

And I'm getting this error:

 ERROR ( hresult:80070005, message:Command execution failed.
 Access is denied.

I am an admin on the box (2012R2/IIS8.5) and I'm running the command prompt as admin. Also I have the Full Control permission (via the local administrators group) on the inetsrv directory. Can someone tell me where else to check?

Thanks,