powershell import-fimconfig- How to set null value to a datetime attribute in FIM portal

I am running a powershell to clear out a datetime attribute in FIM portal. When $import-fimconfig is called, it is throwing the error "Attribute with value does not conform to a valid date time format. Here is the powershell snip. $attributeValue = $NULL. What am I missing?

$importChange = New-Object Microsoft.ResourceManagement.Automation.ObjectModel.ImportChange
    $importChange.Operation = 2
    $importChange.AttributeName = $attributeName
    $importChange.AttributeValue = $attributeValue
    $importChange.FullyResolved = 1
    $importChange.Locale = "Invariant"
 

    $importObject = New-Object Microsoft.ResourceManagement.Automation.ObjectModel.ImportObject
    $importObject.ObjectType = $objectType
    $importObject.TargetObjectIdentifier = $objectID
    $importObject.SourceObjectIdentifier = $objectID
    $importObject.State = 1
    $importObject.Changes = (,$importChange)
    
    $importObject | Import-FIMConfig -uri $uri