Active Directory :: The Attribute Syntax Specified To The Directory Service Is Invalid

I have written a script to update user information in the Active Directory using ADSI. Here is part of it:

Set User = GetObject("LDAP://<GUID=" & GUID & ">")

User.Department = DepartmentName

User.SetInfo

Set User = Nothing

This works fine unless DepartmentName is an empty string ("").

Then I get error 0x8007200B: "The attribute syntax specified to the directory service is invalid."
This happens with all the attributes I have tried, including TelephoneNumber
.
Do I need to delete the value of the attribute instead of setting it to an empty string? If so, how do I do it?