A summary has been specified on a non-recurring field. on a formula

Hello every one,

This is my problem I need to check if the next record/field is equal to the record I'm reading then need to be 0, I evaluated what type of fruit first then record number 12345 example:

numbervar countop;

if {slro.fruit } > 15000 and {slro.fruit } < 22500 then

if previous({slro.fruit}) = {slro.fuit} then countop:=-1 else 0

else countop:=0;

countop

a apple 12345 1

b apple 12345 0

c apple 12345 0

d apple 67890 1

e apple 67891 1

f apple 67892 1

But I get and error A summary has been specified on a non-recurring field on a formula,,, Why?

Any help is welcome Thank you.