Mailbox role install failed

Mailbox Role
Failed

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.
Click here for help... http://technet.microsoft.com/en-US/library/ms.exch.err.default(EXCHG.141).aspx?v=14.1.218.11&e=ms.exch.err.Ex88D115&l=0&cl=cp

Elapsed Time: 00:02:57

Finalizing Setup
Cancelled

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