try putting an if statement at the beginning, something like,
=if(SUMIF(worksheet#1!E1:E15,worksheet#2!A1,worksh eet#1!B1:B15)/COUNTIF(worksheet#1!E1:E15,worksheet#2!A1)),

(SUMIF(worksheet#1!E1:E15,worksheet#2!A1,worksheet #1!B1:B15)/COUNTIF(worksheet#1!E1:E15,worksheet#2!A1)
), "NULL").

what this says is, if function = "something", then "something", else
"NULL".

my syntax may not be exactly right, but this is the idea that i have
used in the past.