diff --git a/js/reveal.js b/js/reveal.js
index 98d802e..b7764d3 100644
--- a/js/reveal.js
+++ b/js/reveal.js
@@ -2032,7 +2032,7 @@ var Reveal = (function(){
 
 			var slideHeight = dom.background.offsetHeight;
 			var verticalSlideCount = verticalSlides.length;
-			var verticalOffset = verticalSlideCount > 0 ? -( backgroundHeight - slideHeight ) / ( verticalSlideCount-1 ) * indexv : 0;
+			var verticalOffset = verticalSlideCount > 1 ? -( backgroundHeight - slideHeight ) / ( verticalSlideCount-1 ) * indexv : 0;
 
 			dom.background.style.backgroundPosition = horizontalOffset + 'px ' + verticalOffset + 'px';