The distribution of the number of parts of $m$-ary partitions modulo $m$

We investigate the number of parts modulo $m$ of $m$-ary partitions of a positive integer $n$. We prove that the number of parts is equidistributed modulo $m$ on a special subset of $m$-ary partitions. As consequences, we explain when the number of parts is equidistributed modulo $m$ on the entire set of partitions, and we provide an alternate proof of a recent result of Andrews, Fraenkel, and Sellers about the number of $m$-ary partitions modulo $m$.
Comment: 10 pages, 1 figure, To appear in Rocky Mountain Journal of Mathematics