Powershell script to update mailbox search

I am trying to update a mailbox search using a powershell script:

Stop-MailboxSearch -Identity "LitHolds" -confirm:$false

$members  = (import-csv -Path "Path"| ForEach-Object{$_.SamAccountName})

foreach ($user in $members) 
{

Set-MailboxSearch -Identity "LitHolds" -SourceMailboxes $user.samaccountname

}

Start-MailboxSearch -Identity "LitHolds" -confirm:$false

I essentially have our sysadmins populating a csv file and then they run this script manually every night with the latest updates.  They are telling me that the mailbox search is empty.

When I run the script I am getting a watson error immediately after with the following:

WARNING: An unexpected error has occurred and a Watson dump is being generated: Value cannot be null.
Parameter name: legacyDN
Value cannot be null.
Parameter name: legacyDN
    + CategoryInfo          : NotSpecified: (:) [Stop-MailboxSearch], ArgumentNullException
    + FullyQualifiedErrorId : System.ArgumentNullException,Microsoft.Exchange.Management.Tasks.StopMailboxSearch
    + PSComputerName        : Exchange Server Name

What I don't understand is why it's looking for a legacyDN??

Anyone able to do something similar?

Chuck