Variables -SearchBase

Im a little lost here.  Hopefully you can see with my code what I am trying to achieve:

$Dept=@("Sales","Finance")
$OU=@("ou=Sales,ou=Finance,ou=WS,dc=ads,dc=contoso,dc=net")
#change -lt value to add more sources and destinations
for ($i=0; $i -lt 2; $i++) {

Write-Host $i

Get-ADUser -SearchBase $OU -Filter * | `
 Set-ADUser -City "XXXX" `
  -State 'XXXX' `
  -StreetAddress 'XXXXXXX' `
  -Description 'XXXX' `
  -Organization 'XXXXXXXX' `
  -Country 'XX' `
  -Department "$Dept" `
  -PostalCode XXXX `
  -PassThru


}

I have replaced the true values with X

I get this error when I run it:

Get-ADUser : Cannot convert 'System.Object[]' to the type 'System.String' required by parameter 'SearchBase'. Specified method is not supported.
At C:\Users\XXX\Desktop\Update_Address_details.ps1:8 char:24
+ Get-ADUser -SearchBase $OU -Filter * | `
+                        ~~~
    + CategoryInfo          : InvalidArgument: (:) [Get-ADUser], ParameterBindingException
    + FullyQualifiedErrorId : CannotConvertArgument,Microsoft.ActiveDirectory.Management.Commands.GetADUser

Any ideas?

Chris Golden