diff --git a/lib/bitmapper.js b/lib/bitmapper.js
index 6fa1823..a55c715 100644
--- a/lib/bitmapper.js
+++ b/lib/bitmapper.js
@@ -98,12 +98,13 @@ exports.dataToBitMap = function(data, width, height, bpp, depth, interlace) {
   for(var imageIndex = 0; imageIndex < images.length; imageIndex++) {
     var imageWidth = images[imageIndex].width;
     var imageHeight = images[imageIndex].height;
+    var imagePass = images[imageIndex].index;
     for (var y = 0; y < imageHeight; y++) {
       for (var x = 0; x < imageWidth; x++) {
         if (depth !== 8) {
           pixelData = bits.get(bpp);
         }
-        var pxPos = getPxPos(x, y, imageIndex);
+        var pxPos = getPxPos(x, y, imagePass);
         //console.log(x,y,imageIndex, pxPos);
         for (var i = 0; i < 4; i++) {
           var idx = pixelBppMap[bpp][i];
diff --git a/lib/interlace.js b/lib/interlace.js
index 47ba8fa..bf867e7 100644
--- a/lib/interlace.js
+++ b/lib/interlace.js
@@ -25,7 +25,7 @@ exports.getImagePasses = function(width, height) {
       }
     }
     if (passWidth > 0 && passHeight > 0) {
-      images.push({ width: passWidth, height: passHeight});
+      images.push({ width: passWidth, height: passHeight, index: i});
     }
   }
   return images;
diff --git a/lib/parser.js b/lib/parser.js
index 72bb3c5..c8b5593 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -323,6 +323,10 @@ Parser.prototype.reverseFiltered = function(indata, depth, width, height) {
         var pxPos = pxRowPos + (x << 2),
           color = this._palette[indata[pxPos]];
 
+        if (!color) {
+          throw new Error("index " + indata[pxPos] + " not in palette");
+        }
+
         for (var i = 0; i < 4; i++)
           indata[pxPos + i] = color[i];
       }