When I try to run the following powershell script:
Add-PSSnapin Microsoft.Sharepoint.Powershell
[System.reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")
$site = new-object Microsoft.SharePoint.SPSite(“http://servername/sites/contoso”)
$web = Get-SPWeb -Identity http://servername/sites/contoso
$lista =$web.Lists["somelist"]
$items = $lista.items
foreach ($item in $items) {
$user = $item["A"]
write-host $useracc
$useracc = $web.EnsureUser( $user);
$item["B"] = $useracc
write-host $useracc
$item.update()
}
$lista.update()
I get the following error:

The source list and destination columns are just plaintext type, so I am puzzled why it is mentioning the user “12” could not be found. 12 is one the a values of the source column.
thanks!