Crystal Reports Formula shows blank records

Message

   I've got four tables of data:
Table A
Username
WidgetApurchased

Table B
Username
WidgetBpurchased

Table C
Username
WidgetCpurchased

TableD
Username
WidgetDpurchased

I want to bring in the widgets purchased from each table to get a Total Widgets Purchased field. Unfortunately, when some WidgetPurchased fields are null, then summation formula also returns null. 

Example:

Username               Widget A          B             C           D             Total
John                           500             233           -           40               -
Peter                         350              400          30          40            820
Paul                             -                 300         300        300            -

How can I make my summation formula still show the summary of the widgets purchased even if one of the widgets purchased categories doesn't show anything?

Thanks for the help!