formula returns wrong result

Have this code in cell E21

 =SUM(C21-D21)*(MATCH(B21,$AC$25:$AC$44,0)*5)

works
if I have number in D21 perfect. How can I make it so if D21 has no number E21 will show 0. Now it picks up Sum of C21 and fills
E21 with number even id D21 has no number. Not sure how to make formula return 0 if D21 is empty?.


Thanks z
Z