Joplin's "The Entertainer"—is this a misprint?

Question

I've just started learning Scott Joplin's "The Entertainer", and I think my edition must have a misprint. I'm hoping someone can double-check and confirm my suspicion.

In measure 18, in the left hand, on the second beat, the sheet music shows an A♭ octave followed by a chord spelled G-C-F (see the photo). That chord has an open-fourth sound which sounds out of place in Joplin's music. I figure the bottom G's gotta be an A♭, making the chord an F-minor inversion. Can anyone confirm? Or is the printed music, in fact, correct?

Thanks!

Answer 1

This is a misprint. It should be an Ab, just as you've said. If you look here, you'll see the Ab present:

¹

And if you listen to the recording below, you'll hear the Ab at ~0:49 and ~1:59. Slowing down to 50% of the original video speed can help.

¹http://fileserver.freehandmusic.netdna-cdn.com/preview/530x4/joplin/jsenterr.png