diff --git a/src/zm_image.cpp b/src/zm_image.cpp
index 8c7189e6b..3fbb458f5 100644
--- a/src/zm_image.cpp
+++ b/src/zm_image.cpp
@@ -2720,9 +2720,12 @@ void Image::Fill(Rgb colour, int density, const Polygon &polygon) {
     std::sort(active_edges.begin(), active_edges.end(), PolygonFill::Edge::CompareX);
 
     if (!(scan_line % density)) {
-      for (auto it = active_edges.begin(); it < active_edges.end() - 1; ++it) {
+      // Fill between pairs of active edges (parity rule). Stepping one
+      // edge at a time would incorrectly fill the gaps between arms of
+      // a non-convex polygon (e.g. a banana shape).
+      for (auto it = active_edges.begin(); it + 1 < active_edges.end(); it += 2) {
         int32 lo_x = static_cast<int32>(it->min_x);
-        int32 hi_x = static_cast<int32>(std::next(it)->min_x);
+        int32 hi_x = static_cast<int32>((it + 1)->min_x);
         if (colours == ZM_COLOUR_GRAY8) {
           uint8 *p = &buffer[(scan_line * width) + lo_x];