relevant calculated fields are returning 0.00 when there is nothing to calculate and should be NULL

[LynnBerni]

In the following screenshot, the fields 5 & 6 are calculated based on data in fields 1-4. The whole group (fields1-6) is relevant as to whether or not a sample is collected. But, when no sample is collected, the calculated fields are returning 0.00. I would like them to be blank. Is there a way to control (cancel) the calculation when the relevant expression is not met?

[LynnBerni]

dfb in egdb

[SteveLynch]

shapefile, which uses .dbf, converts null to zero.

To test, create a field in a fgdb that has nulls. Use CopyFeatures and save it to shapefile and you'll notice the zeros.

[DanPatterson_Retired]

Lynn, it sounds like relying on null values should be avoid in that case.  If the data you are using are counts, put in a count value that isn't possible... you could go 'old school' and use -999 or some equivalent scaled variant.  Any 

observation which is a count cannot possibly have a negative number

[LynnBerni]

Thanks Dan, that's an excellent point!