I would need help in order not to mix up de and le or un
I came across this sentence
Il n'y a pas de bus direct?
But I couldn't understand why le wasn't used (or for that matter un, if you don't want to refer to a particular bus). Y avoir doesn't seem to be in this list of verbs that are succeeded by «de» here http://french.about.com/od/grammar/a/preposition_de_3.htm