Field cannot be summarized

I have created a formula field called "other" with the following:
if Count({BOR9798B.SSN},{bormatch9798984.NAICS}) < 6 then
{@.freq} else 0
When I create a formula field called "other2" with the following:
Sum ({@.Other}) I get the message 'This field cannot be summarized'. What can I do?What is the datatype of that field?