How to round a double8 data type in sql?

Postgres 8.4

How do I use round() in sql on a double8 data type? While in an SQL
statement, I can use round just fine on a field that is a type of double.
Ex: Select round(esthours,2) as ehrs from job;

But a different field has a type of double8 and I get an error when I try
to use round on it:

select round(acthours,2) as ahrs from job;

The error is: "No function matches the given name and argument types. You
might need to add explicit type casts."

I searched around on Google but found no help on doing type casts.

Thank you.