I just wanted to chime in, since previous discussion above or below seems not to have affected this yet, that if it's within their powers the mods could adjust this sentence to allow "a" or "an" equally so that "a" would not be marked as a typo. It's definitely not a typo in American English, in which both forms are accepted but "an" comes across as stuffy, excessively proper, or foreign.
But it isnt for a person speaking correct English. It is natural to us, and the fact that it seems strange shows the degree that English had been compromised. That is not wrong in itself.. All languages evolve and that is why English is so successful and not held back by an Academie Nationale which decrees what is permitted. But we are close to a time when English and American should be taught and studied seperately. Incidentally. An Historian is also correct.