SQL & PL/SQL

Hi, I need to use the utl_url.escape function to convert some illegal characters in a url and I want to know why I get the error "FALSE: invalid identifier" when I use the function like this:
SELECT utl_url.escape('http://www.acme.com/a url with space.html', FALSE, 'UTF8')
FROM dual;
Where the first parameter is the url I want to escape, the second one indicates that I don't want to escape the reserved chars, and the third one is the target charset. I search in the documentation and this is a right way to use the function.
If I use the function without the second and third arguments, it work properly but it don't make a good conversion, I think it is because I am not indicating the target charset.
Any help will be appreciated.