Revoking Function Execute Privilege

I've executed the following in a clean database:

As postgres/superuser:

CREATE ROLE impotent NOLOGIN;

CREATE FUNCTION testfunc() RETURNS boolean AS $$

BEGIN

RETURN true;

END;

$$ LANGUAGE 'plpgsql';

REVOKE ALL ON FUNCTION testfunc() FROM impotent;

SET ROLE impotent;

SELECT has_function_privilege('impotent','testfunc()','execute');

SELECT testfunc();

The has_function_privilege returns true instead of false and the SELECT
testfunc() returns without an exception.

I expected failure due to the REVOKE ALL . FROM impotent so what am I
missing?

Thanks,

David J