diff --git a/lib/cartodb/controllers/layergroup.js b/lib/cartodb/controllers/layergroup.js
index 6711b401..9838838b 100644
--- a/lib/cartodb/controllers/layergroup.js
+++ b/lib/cartodb/controllers/layergroup.js
@@ -435,7 +435,7 @@ function getPreviewImageByCenter (previewBackend) {
         const { mapConfigProvider: provider } = res.locals;
 
         previewBackend.getImage(provider, format, width, height, zoom, center, (err, image, headers, stats = {}) => {
-            req.profiler.done('render-' + format);
+            req.profiler.done(`render-${format}`);
             req.profiler.add(stats);
 
             if (err) {