Exchange 2010 PowerShell script throwing errors on specific users.

I have been given the task to track the size of each users inbox, deleted items, sent items, and calendar. I do not find much on the googles for reporting just inbox size, rather most IT seem to report the entire mailbox size. I did however find and slightly modify the script below that works perfectly on 776/822 of our users. with the other 46 users I get the error below. I have tried for longer then I am willing to admit trying to get this script and others to work. Possibly there is a different way to loop this, I can manually pull information from the users, just not with the scripting methods I have tried.  

You cannot call a method on a null-valued expression.
At line:10 char:127
+            $mbObj | Add-Member -MemberType NoteProperty -Name "Inbox Size (Mb)" -Value $inboxstats.FolderandSubFolder
Size.ToMB <<<< ()
    + CategoryInfo          : InvalidOperation: (ToMB:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull



The error repeats for all 4 boxes. Here is the script I am using



$mailboxes = @(Get-Mailbox -ResultSize Unlimited)
$report = @()

foreach ($mailbox in $mailboxes)
    {
          $inboxstats = Get-MailboxFolderStatistics $mailbox -FolderScope Inbox | Where {$_.FolderPath -eq "/Inbox"}
          $deletedstats = Get-MailboxFolderStatistics $mailbox | Where {$_.FolderPath -eq "/Deleted Items"}
          $sentitemstats = Get-MailboxFolderStatistics $mailbox | Where {$_.FolderPath -eq "/Sent Items"}
          $calendarstats = Get-MailboxFolderStatistics $mailbox | Where {$_.FolderPath -eq "/Calendar"}

           Write-Host "Getting Stats for " $mailbox.DisplayName -ForegroundColor Green

           $mbObj = New-Object PSObject
           $mbObj | Add-Member -MemberType NoteProperty -Name "Display Name" -Value $mailbox.DisplayName
           $mbObj | Add-Member -MemberType NoteProperty -Name "Inbox Size (Mb)" -Value $inboxstats.FolderandSubFolderSize.ToMB()
           $mbObj | Add-Member -MemberType NoteProperty -Name "Deleted Items(MB)" -Value $deletedstats.FolderAndSubfolderSize.ToMB()
           $mbObj | Add-Member -MemberType NoteProperty -Name "Sent Items(MB)" -Value $sentitemstats.FolderAndSubfolderSize.ToMB()
           $mbObj | Add-Member -MemberType NoteProperty -Name "Calendar(MB)" -Value $calendarstats.FolderAndSubfolderSize.ToMB()

           $report += $mbObj

      }

$report | Export-Csv c:\Temp\Report.csv -NoTypeInformation