Trigger problem

Hi friends!

I'm working in a trigger and I need to put the result
of a query into a variable.

That's very easy- apparently!

The query has a aggregate function like this:

select sum(field) into variable ...

and I'm sure that field and variable are int4 type.

So, when I run this trigger there is a mistake:
 ''there is no operator '=$' for types 'int4' and 'int4'
   you will either have to retype this query using an
   explicit cast, or you will have to define the operator
   using CREATE OPERATOR''

what's meaning this? and
how can I assign the result of aggregate function into a
variable?
(My system is 6.5.3)