Exchange Server 2010 - Setup /PrepareSchema Fails

I am installing an exchange server 2010 on Windows server 2008 R2. When attempting to run setup /PrepareSchema, the following error occurs:

No key presses were detected.  Setup will continue.
Preparing Exchange Setup

    Copying Setup Files                           COMPLETED

No server roles will be installed

Performing Microsoft Exchange Server Prerequisite Check

    Organization Checks                                       COMPLETED

Configuring Microsoft Exchange Server

    Extending Active Directory schema                         FAILED
     The following error was generated when "$error.Clear();
          if ($RoleDomain -ne $null)
          {
          initialize-ExchangeLegacyPermissions -Domain $RoleDomain -DomainContro
ller $RoleDomainController;
          }
          else
          {
          initialize-ExchangeLegacyPermissions -AllDomains:$true -DomainControll
er $RoleDomainController;
          }
        " was run: "Object reference not set to an instance of an object.".


The Exchange Server setup operation didn't complete. More details can be found
in ExchangeSetup.log located in the <SystemDrive>:\ExchangeSetupLogs folder.

Exchange Server setup encountered an error.