"Case" calculation is returning no result
I've set up a calculation field for an ESS relational database; it's a straightforward "Case" calculation returning numeric values from 0 to 6, defaulting to 7. It's working as expected for most records in the external database, returning the appropriate values -- but there are a number of records where the field is blank. How can this be? The "Case" statement is set up so that, if none of the stated conditions are true, the default value is returned -- but in these records nothing is returned. What am I doing wrong?