Updating user attributes from CSV in powershell

I have a script to update user attributes by fetching them from a csv file. However it gives me below mentioned error on every attribute. Please help.

ERROR:

======================================================================================================

Set-ADUser : Cannot find an object with identity: '' under: 'DC=parco,DC=com,DC=pk'.
At C:\Users\zeeshan.opel\Desktop\UserUpdate.ps1:27 char:5
+ Set-ADUser -Identity "$ADUser" -Replace @{postOfficeBox=$POBox}
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (:ADUser) [Set-ADUser], ADIdentityNotFoundException
+ FullyQualifiedErrorId : ActiveDirectoryCmdlet:Microsoft.ActiveDirectory.Management.ADIdentityNotFoundException,Microsoft.Activ
eDirectory.Management.Commands.SetADUser

====================================================================================================== 

SCRIPT:

# Import AD Module
Import-Module ActiveDirectory
# Import User Details from CSV
$users = Import-Csv -Path C:\users.csv
# Get Values from CSV in Seperate Variables
foreach ($user in $users)
{
$Acc = $users.samaccountname
$ID = $users.employeeID
$Company = $users.company
$Grade = $users.grade
$Title = $users.title
$Division = $users.division
$Department = $users.department
$City = $users.city
$POBox = $users.pobox

# Find out of user from domain id
$ADUser = Get-ADUser -Filter "samaccountname -eq '$Acc'"
# Set user properties according to DN
Set-ADUser -Identity "$ADUser" -Replace @{employeeID=$ID}
Set-ADUser -Identity "$ADUser" -Replace @{title=$Title}
Set-ADUser -Identity "$ADUser" -Replace @{division=$Division}
Set-ADUser -Identity "$ADUser" -Replace @{company=$Company}
Set-ADUser -Identity "$ADUser" -Replace @{department=$Department}
Set-ADUser -Identity "$ADUser" -Replace @{l=$City}
Set-ADUser -Identity "$ADUser" -Replace @{postOfficeBox=$POBox}
}