diff --git a/src/context.js b/src/context.js
index d6c52f70..58b9455c 100644
--- a/src/context.js
+++ b/src/context.js
@@ -291,7 +291,7 @@
   }
 
   window.onload = function() {
-    if (oldWindowLoad) {
+    if (typeof oldWindowLoad === 'function') {
       oldWindowLoad()
     }
     Context.refreshAll()