Actually, I think saying "part of" may be the informal way and you do have to include the "a" in it. I can't be too sure, though. If it's not then all I can suggest is that maybe it has to do with whether the pronoun is singular or plural. I mean, "our" is plural, and the word "a" was excluded. Then again, it would be correct even if you did place the "a" in there.