#DIV/0! error when data is blank

Im trying to build a spreadsheet on data obtained from patients. I would like to express as a percentage of data captured.  I have already set the constants to numbers only, but when I clear a cell (because there is no data yet), my percentage cell gives a #DIV/0! error. My formula is =SUM(C5,D5,F5)/B5. Additionally, B5's formula is =SUM(C5:F5). Thanks for any help.