Access: Dcount number of records where field is Null

I submitted a string of code below: it is supposed to search my table ("StudentName") for all records where the field "NumberStudents" is null and StudentName = "Tom". I can not get the "Null" part to work. (password is always 0)Will someone correct my syntax below so that my code will work.

dim password as integer

password = DCount("[NumberStudents]", "[Classes]", "[StudentName] = "Tom"' and [NumberStudents] ='" & Null & "'")
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