diff --git a/lib/browser/startAnalyzing.js b/lib/browser/startAnalyzing.js
index 53b1e87..85036b3 100644
--- a/lib/browser/startAnalyzing.js
+++ b/lib/browser/startAnalyzing.js
@@ -41,7 +41,11 @@ module.exports = function(_, pcSelectorMethod, interval) {
      * sanitize stat values
      */
     function sanitize(val) {
-        return parseInt(val || 0, 10);
+        if(typeof val !== 'string' && typeof val !== 'number') {
+            return undefined;
+        }
+
+        return parseInt(val, 10);
     }
 
     /**