Ok. So what are you trying to accomplish? Do you just want nothing to appear on the report if a number is 0? If that's the case, instead of converting to null, you'll conditionally suppress the object on the report.
Right-click on the object, select "Format...". On the Common tab, click on the button to the right of the word "Suppress" (DO NOT check the checkbox!) Enter the following in the formula:
-Dell
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