Indexes on Expressions -- Parentheses

From 11.5 in the docs:

"The syntax of the CREATE INDEX command normally requires writing
parentheses around index expressions, as shown in the second example.
The parentheses may be omitted when the expression is just a function
call, as in the first example."

But when I try this:

db=# CREATE INDEX expression_idx on some_table( extract( year from
some_column ) );

I get a syntax error corresponding to the open parenthesis after
extract (and whitespace is not the issue).

If I add an enclosing set of parentheses, E.g.:

db=# CREATE INDEX expression_idx on some_table( ( extract( year from
some_column ) ) );

it works!

Is this a known issue?

-tfo