[link to original on tumblr]
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It's not. It's A minor and Asus4.
In the rough version I just posted, I played it with barre chords, but it may have been easier to do it with the "normal" version of Am. I just have a tendency to play barre chords because - despite their being harder sometimes - I feel they give a more even sound. I could go into a whole explanation of how barre chords help to fix the uneven distribution of chords' constituent notes (both in position and number) that occurs in standard tuning (which is one reason why I love open D tuning - it doesn't have that problem), but I'll just leave it that.