diff --git a/include/mapnik/octree.hpp b/include/mapnik/octree.hpp
index 67eed7637..40c2dc5d6 100644
--- a/include/mapnik/octree.hpp
+++ b/include/mapnik/octree.hpp
@@ -122,6 +122,11 @@ namespace mapnik {
 
          ~octree() { delete root_;}
 
+         unsigned colors()
+         {
+            return colors_;
+         }
+
          void setMaxColors(unsigned max_colors)
          {
             max_colors_ = max_colors;
diff --git a/include/mapnik/png_io.hpp b/include/mapnik/png_io.hpp
index ae25aef7d..b78e15432 100644
--- a/include/mapnik/png_io.hpp
+++ b/include/mapnik/png_io.hpp
@@ -131,7 +131,7 @@ namespace mapnik {
             byte index = 0;
             int idx = -1;
             for(int j=levels-1; j>0; j--){
-               if (U2ALPHA(val)>=limits[j]) {
+               if (U2ALPHA(val)>=limits[j] && trees[j].colors()>0) {
                   index = idx = trees[j].quantize(c);
                   break;
                }
@@ -177,7 +177,7 @@ namespace mapnik {
             byte index = 0;
             int idx=-1;
             for(int j=levels-1; j>0; j--){
-               if (U2ALPHA(val)>=limits[j]) {
+               if (U2ALPHA(val)>=limits[j] && trees[j].colors()>0) {
                   index = idx = trees[j].quantize(c);
                   break;
                }