Good morning Nic
Can't tell without seeing your data but there are 2 types of nulls - one where there is a record with no data in it - and the other where there is no record for the combination of dimensions in the viz
the 3 records in the rows 2-4 are an example of the second type - isnull will not find them - try using a lookup function like - ifnull(lookup(dimension),0,'Your string value')
Lookup is a table calculation that you will have to set up correctly depending on the dimensions in your viz
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Thanks Jim, your correct I'd been playing with my Data Source.