Writing data test and expecting null values

How do we write data tests to ensure the return are always null? I have some filter conditions for a certain set of results that should never appear. I have made sure to use a count measure and make it = 0.  In this example below, count_id is a measure. Looks correct when I run it in the explore as I am indeed expecting zero rows.

assert: customer_id_more_than_one_is_zero_1 {
    expression: (${count_id} = 0 ) ;;
  }

However this is the error I get which doesn't make sense as my assert looks right. 

Every test query must return at least one row. If you want to assert a result set is empty, use a count field and assert it is zero.

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