diff --git a/include/mapnik/png_io.hpp b/include/mapnik/png_io.hpp
index f01492198..a17da9b35 100644
--- a/include/mapnik/png_io.hpp
+++ b/include/mapnik/png_io.hpp
@@ -684,7 +684,7 @@ void save_as_png8_hex(T1 & file,
     //transparency values per palette index
     std::vector<mapnik::rgba> pal;
     tree.create_palette(pal);
-    assert(int(pal.size()) <= colors);
+    assert(int(pal.size()) <= opts.colors);
     std::vector<mapnik::rgb> palette;
     std::vector<unsigned> alphaTable;
     for(unsigned i=0; i<pal.size(); i++)