Exchange2010 fails to install Mailbox Role "Cannot bind argument to parameter 'Identity' because it is null"

I'm getting the below error trying to install Exchange 2010 mailbox role on a server that already has HT and CAS roles installed successfully.

Environment:

32-bit 2003 R2 domain controllers

domain in 2003 native mode

existing, functional exchange 2003 server in environment.

fresh server 08 r2 x64 member server with pre-requisites installed.

/prepareschema /preparead and /pad all worked fine

The error

---------------------------

Error:
The following error was generated when "$error.Clear();
          if ($RoleCreatePublicFolderDatabase)
          {
            $publicDB = get-PublicFolderDatabase -Server:$RoleFqdnOrName -ErrorAction SilentlyContinue;
            $DB = get-MailboxDatabase -Server:$RoleFqdnOrName -ErrorAction SilentlyContinue;
            if ($publicDB -and $DB)
            {
                set-mailboxdatabase `
                  -Identity:$DB.Identity `
                  -publicFolderDatabase:$publicDB.Identity `
                  -DomainController $RoleDomainController
            }                 
          }
        " was run: "Cannot bind argument to parameter 'Identity' because it is null.".

Cannot bind argument to parameter 'Identity' because it is null.

Elapsed Time: 00:01:09

Finalizing Setup
Cancelled
----------------------------------

any help would be much appreciated