“An R package” or “A R package”

I’m currently writing some lecture notes on R and I used the phrase “a R package” without thinking. Since the word following the article “a” was a consonant, I automatically went for “a” instead of “an”. The problem is that “R” sounds likes a vowel, so “a R package” grates on the listener. The correct rule is to use “an” when the word following the article “sounds like a vowel”.

A quick google search suggests other people mess up too:

• “an R package” – around 600, 000 hits;
• “a R package” – around 150,000 hits.

Who would have thought that people could be wrong on the internet…