SQL IS NULL error

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Hi 

We're trying to create multiple reports where we want to bring back a column with no data in (e.g. Missing Bank Details where the employee does not have a bank account number). When we run the report without a filter, we get back all the results as expected (records who have bank details and records who do not have bank details). When we apply the IS NOT NULL filter, we get back all the results as expected; records whom have bank details. However, when we apply the IS NULL filter, we get an error and don't get back any results