Add null value

Question

I need to update several AD user accounts with the following:
Get-ADUser -Filter * -SearchBase 'ou=Active,ou=Students,OU=People,dc=domainr,dc=com' -Properties userPrincipalName | foreach { Set-ADUser $_ -UserPrincipalName ("{0}@{1}" -f $_.name,"domain.com")}
This works fine, but I only want to update the records that are empty/blank.  How do I apply a search/apply for only those with a null value?
mdclem

Answer 1

Fausto and Clayman2, thank you for replying.  I've added the changes:
Get-ADUser -Filter * -SearchBase 'ou=Active,ou=Students,OU=People,dc=domain,dc=com' -Properties userPrincipalName -Filter "userPrincipalName -eq $null"| Set-ADUser -Whatif -UserPrincipalName ("{0}@{1}" -f $_.name,"domain.com")
But now get the following error:
PS C:\Windows\system32> Get-ADUser -Filter * -SearchBase 'ou=Active,ou=Students,OU=People,dc=domain,dc=com' -Properties userPrincipalName -Filter "userPrincipalName -eq $null"| Set-ADUser -Whatif -UserPrincipalName ("{0}@{1}" -f $_.name,"domain.com")
Get-ADUser : Cannot bind parameter because parameter 'Filter' is specified more than once. To provide multiple values to parameters that can accept multiple values, use the array syntax. For example, "-parameter value1,value2,value3".
At line:1 char:114
+ ... rPrincipalName -Filter "userPrincipalName -eq $null"| Set-ADUser -Whatif -UserPr ...
+                    ~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [Get-ADUser], ParameterBindingException
    + FullyQualifiedErrorId : ParameterAlreadyBound,Microsoft.ActiveDirectory.Management.Commands.GetADUser
Where did I mess this up?
mdclem