error selecting null columns

I want to run a query that selects rows from the table where a datetime column has null values;

select*from OrderswhereIsNull(dClosedDate,'Null')='Null'

However i get this error:

Conversion failed when converting datetime from character string.

Any help appreciated

SELECT * FROM Orders WHERE ISNULL(dClosedDate, '1900-01-01') = '1900-01-01'

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thx...i tried dClosedDate is NULL and it worked too.