#DIV/0! Errors?

I have created a basic spreadsheets that show sales in column b and profit is column c and totals in column d I want it to divide profit by sales to give a gp% which appears in column d but when I have either nothing or a 0 in any of the bands b or c columns I get an error #DIV/0! in column d? I also have it averaging at the bottom of column d with the same results how can I stop this error appearing please because many weeks we have no figures yet so they are blank?

Any advice would be appreciated.