diff --git a/src/ol/source/singleimagewmssource.js b/src/ol/source/singleimagewmssource.js
index ec4f9a199d..d0a1ff88f4 100644
--- a/src/ol/source/singleimagewmssource.js
+++ b/src/ol/source/singleimagewmssource.js
@@ -1,9 +1,9 @@
 goog.provide('ol.source.SingleImageWMS');
 
-goog.require('ol.Extent');
 goog.require('ol.Image');
 goog.require('ol.ImageUrlFunction');
 goog.require('ol.Size');
+goog.require('ol.extent');
 goog.require('ol.source.ImageSource');
 goog.require('ol.source.wms');
 
@@ -56,15 +56,14 @@ ol.source.SingleImageWMS.prototype.getImage =
   var image = this.image_;
   if (!goog.isNull(image) &&
       image.getResolution() == resolution &&
-      image.getExtent().containsExtent(extent)) {
+      ol.extent.containsExtent(image.getExtent(), extent)) {
     return image;
   }
 
-  extent = new ol.Extent(extent.minX, extent.minY,
-      extent.maxX, extent.maxY);
-  extent.scaleFromCenter(this.ratio_);
-  var width = extent.getWidth() / resolution;
-  var height = extent.getHeight() / resolution;
+  extent = extent.slice();
+  ol.extent.scaleFromCenter(extent, this.ratio_);
+  var width = (extent[1] - extent[0]) / resolution;
+  var height = (extent[3] - extent[2]) / resolution;
   var size = new ol.Size(width, height);
 
   this.image_ = this.createImage(extent, resolution, size, projection);