diff --git a/lib/interlace.js b/lib/interlace.js
index cc76542..734ea23 100644
--- a/lib/interlace.js
+++ b/lib/interlace.js
@@ -48,7 +48,7 @@ exports.getImagePasses = function(width, height) {
   var xLeftOver = width % 8;
   var yLeftOver = height % 8;
   var xRepeats = (width - xLeftOver) / 8;
-  var yRepeats = (width - yLeftOver) / 8;
+  var yRepeats = (height - yLeftOver) / 8;
   for (var i = 0; i < imagePasses.length; i++) {
     var pass = imagePasses[i];
     var passWidth = xRepeats * pass.x.length;
diff --git a/test/in/gh-21.png b/test/in/gh-21.png
new file mode 100644
index 0000000..24a4097
Binary files /dev/null and b/test/in/gh-21.png differ
diff --git a/test/index.html b/test/index.html
index fa20506..6b8a0cb 100644
--- a/test/index.html
+++ b/test/index.html
@@ -198,6 +198,7 @@
 <p class="testline"><img class="inimage" src="in/g03n3p04.png"> <img class="asyncimage" src="out/g03n3p04.png"> <img class="syncimage" src="outsync/g03n3p04.png"> </p>
 <p class="testline"><img class="inimage" src="in/g03n2c08.png"> <img class="asyncimage" src="out/g03n2c08.png"> <img class="syncimage" src="outsync/g03n2c08.png"> </p>
 <p class="testline"><img class="inimage" src="in/g03n0g16.png"> <img class="asyncimage" src="out/g03n0g16.png"> <img class="syncimage" src="outsync/g03n0g16.png"> </p>
+<p class="testline"><img class="inimage" src="in/gh-21.png"> <img class="asyncimage" src="out/gh-21.png"> <img class="syncimage" src="outsync/gh-21.png"> </p>
 <script>
     if (!Function.prototype.bind) {
         Function.prototype.bind = function(oThis) {