Display Control Source OR Default Value In Textbox

Table1 has field (% Completed this Period) that may contain a value, or may be NULL for each entry, depending on how much work was performed that period. The records are sorted by date.

Form1 has a textbox that basically keeps a running tally of "% Completed to Date" by summing the "% Completed this Period" for the current period and all previous periods. So, my Control Source for the textbox is the DSum(...) function.

The problem is this: If I am looking at the first record and there is a 0 (null) value for "% Completed this Period," (i.e. No work was completed that period), I want to display 0 on Form1. However, Access just displays nothing - a blank textbox. I tried making a default value, but nothing will show up since I already have a Control Source set for that field. If I take the DSum function out of the Control Source, then the 0 will show up. But, obviously that defeats the whole purpose of the textbox. I guess I'm trying to figure out how to display the DSum value IF it is something other than 0, BUT IF IT IS 0, then I want 0 to be displayed. Any help would be appreciated.