Set-ADUser cmdlet and -AccountExpirationDate

$Expires = $null
Set-ADUser -Identity $DN -AccountExpirationDate $Expires

Error I get:  "Set-ADUser : Missing an argument for parameter 'AccountExpirationDate'. Specify a parameter of type 'System.Nullable`1[System.DateTime]' and try again."

But if I do the following it works: 

Set-ADUser -Identity $DN -AccountExpirationDate $null

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So the reason this is an issue is that I have a number of switch and if statements that set the variables above that line of code and then I run one long Set-ADUser command.  I didn't want to create entirely separate commands for one situation and use another for the other.

So how can I pass a NULL variable that I create?  Or of course any other solution or suggestion...

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