"is not distinct from" syntax error?

From: http://www.postgresql.org/docs/8.2/static/functions-comparison.html

" The ordinary comparison operators yield null (signifying "unknown")
when either input is null. Another way to do comparisons is with the
IS [ NOT ] DISTINCT FROM construct:

expression IS DISTINCT FROM expression
expression IS NOT DISTINCT FROM expression

For non-null inputs, IS DISTINCT FROM is the same as the <> operator.
However, when both inputs are null it will return false, and when just
one input is null it will return true. Similarly, IS NOT DISTINCT FROM
is identical to = for non-null inputs, but it returns true when both
inputs are null, and false when only one input is null. Thus, these
constructs effectively act as though null were a normal data value,
rather than "unknown"."

However, when I do:
SELECT    1 IS NOT DISTINCT FROM 2

I get "ERROR: syntax error at or near "DISTINCT""

I can do:
SELECT    NOT (1 IS DISTINCT FROM 2)

What is the problem here?

Regards,
Collin