--- third_party/agg23/agg_rasterizer_scanline_aa.h
+++ third_party/agg23/agg_rasterizer_scanline_aa.h
@@ -349,14 +349,14 @@
                     cover += cur_cell->cover;
                 }
                 if(area) {
-                    alpha = calculate_alpha((cover << (poly_base_shift + 1)) - area, no_smooth);
+                    alpha = calculate_alpha(int(unsigned(cover) << (poly_base_shift + 1)) - area, no_smooth);
                     if(alpha) {
                         sl.add_cell(x, alpha);
                     }
                     x++;
                 }
                 if(num_cells && cur_cell->x > x) {
-                    alpha = calculate_alpha(cover << (poly_base_shift + 1), no_smooth);
+                    alpha = calculate_alpha(unsigned(cover) << (poly_base_shift + 1), no_smooth);
                     if(alpha) {
                         sl.add_span(x, cur_cell->x - x, alpha);
                     }