As far as I'm concerned, and it stands it seems the most recent post here was five years ago, the proper answer is that only "a" would be appropriate. Many may be confused or feel as though both "a" and "an" would be correct, however this rule applies only to the phonetic spelling, or how it sounds, of the word in question. This same rule also applies when an "o" makes a "y" sound such as in "one". You would say: "a unicycle" and "an umbrella" or "a one-track mind" and "an Oedipus Complex" respectively. The rule is precisely as follows: [The choice of article is based upon the phonetic (sound) quality of the first letter in a word, not on the orthographic (written) representation of the letter. If the first letter makes a vowel-type sound, you use "an"; if the first letter would make a consonant-type sound, you use "a." The exception to this rule is upon the use of the letter "h" as in "an honor student".]
As far as the exception is concerned, I believe this is because the English language does differentiate between global regions and this makes it that it can be spoken in slightly varying manners. I bring to question a word such as "herb". If one were to pronounce the "h" then they would surely say or write "a herb", where as in the United States most people do not pronounce the "h" and they would say or write "an herb (-erb)".
I will note however, that the English language is a malleable one and English speaking societies can change and adapt according to social paradigms. So I fundamentally believe that if you personally feel comfortable in essentially setting your own rules as to how the language is spoken or written, then do so and it is the responsibility of others to decide for themselves if they will adapt your rules or the rules set by the notable English institutions of the past. It really doesn't matter in the grand scheme of things. :)