That's why dictionaries and folks talking about pronunciation ought to use the IPA (link), though the latter often fight its adoption. The first a in asseverate is a schwa /ə/ (link). I have observed that most ad hoc pronunciation "systems" in the States use uh (as did Garu) to represent this sound, but in the UK they use er. This sometimes leads to confusion.

Ceci n'est pas un seing.