Extra empty month printed for every year

[robinadr]

From https://wordpress.org/support/topic/13-empty-month-instead-of-12?replies=2:

I'm currently working with your plugin and I noticed that the plugin print 13 empty-month and not 12. Which cause to display an empty span that have the same style as the other empty-month. My empty month have border and padding so the one that is really empty show up in the list.

Any tips on how to remove that 13th empty-month?

Thank you!