Issue while creating columns in document libraries using powershell

I am using below script to iterate through each document library and create a column with the value equals to the name of library itself:

   $ver = $host | select version 

if($Ver.version.major -gt 1) {$Host.Runspace.ThreadOptions = "ReuseThread"} 

if(!(Get-PSSnapin Microsoft.SharePoint.PowerShell -ea 0)) 

{ 

Write-Progress -Activity "Loading Modules" -Status "Loading Microsoft.SharePoint.PowerShell" 

Add-PSSnapin Microsoft.SharePoint.PowerShell 

} 





$SourceWebURL = "http://sitecollection/sites/sitename" 





$ssite = Get-SPSite $SourceWebURL

$sweb = $ssite.OpenWeb("Op")



foreach($slist in $sweb.Lists) 

{

    if($slist.BaseType -eq "DocumentLibrary")

            {

                        $spFieldType = [Microsoft.SharePoint.SPFieldType]::Text

                        $sList.Fields.Add("OpCo",$spFieldType,$true)

                        $field = $sList.Fields["OpCoName"]

                        $field.DefaultValue = $slist.Title

                        $field.Update()

                        $sList.Update()

            }

}

I am getting below error:

An error occurred while enumerating through a collection: Collection was modifi
ed; enumeration operation may not execute..
At C:\Users\abc.ps1:31 char:8
+ foreach <<<< ($slist in $sweb.Lists)
    + CategoryInfo          : InvalidOperation: (Microsoft.Share...on+SPEnumer
   ator:SPEnumerator) [], RuntimeException
    + FullyQualifiedErrorId : BadEnumeration