can you attach a packaged workbook? that will help us provide better answers. Depending on how your data is structured the approach may be different
But my best guess is that you will have to replace null values with 0 (for numeric fields) or "" (for strings), but if your data is structured how I think, this might be a little tricky.
Thanks for replying, I will need to create some dummy data first as I'm working with live data.