diff --git a/lib/image/png.coffee b/lib/image/png.coffee
index ac9f1f0..11090ed 100644
--- a/lib/image/png.coffee
+++ b/lib/image/png.coffee
@@ -34,7 +34,7 @@ class PNGImage
       @obj.data['ColorSpace'] = @image.colorSpace
     else
       # embed the color palette in the PDF as an object stream
-      palette = document.ref()
+      palette = @document.ref()
       palette.end new Buffer @image.palette
 
       # build the color space array for the image