While generally true that "an" goes before vowels and "a" goes before consonants, it actually matters on how the consonant is pronounced - yeah, I know, English

While generally true that "an" goes before vowels and "a" goes before consonants, it actually matters on how the consonant is pronounced - yeah, I know, English is funny like this. So in this particular case you can use "an" and "a" depending on how you pronounce the letter H.