How to make VBA query for an empty field

Hi,

I am trying to find fields in a query that are empty or null.  Here is an example:

SELECT tblPOHeader.ManufacturerID, tblPOHeader.POHeaderID, tblPOHeader.PVID
FROM tblPOHeader
WHERE (((tblPOHeader.PVID)=IsNull));

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What do I put in the place of "IsNull" in line 3?
Thank you for your help.
Chad