Multiple columns w/single aggregate

I cannot successfully execute the sql statement:

SELECT percentage, MAX(start_date) FROM sometable
WHERE id = 1

I get the error message:

ERROR: Attribute sometable.percentage must be GROUPed
or used in an aggregate function

I know that I have seen examples of selecting multiple columns with a
single colunn aggregate function. In fact, the new 'PostgreSQL:
Introductions and Concepts' book by Bruce Momjian has two on page 53.
Does anyone know why this doesn't work?

Thanks in advance.