diff --git a/src/General/Main.coffee b/src/General/Main.coffee
index 5582ffdfe..95333e029 100644
--- a/src/General/Main.coffee
+++ b/src/General/Main.coffee
@@ -155,7 +155,7 @@ Main =
       $.addClass doc, style
     setStyle()
     return unless mainStyleSheet
-    if MutationObserver
+    if window.MutationObserver
       observer = new MutationObserver setStyle
       observer.observe mainStyleSheet,
         attributes: true
diff --git a/src/Posting/QR.coffee b/src/Posting/QR.coffee
index 1c260737a..dec55d596 100644
--- a/src/Posting/QR.coffee
+++ b/src/Posting/QR.coffee
@@ -703,7 +703,7 @@ QR =
         img:       imgContainer.firstChild
         input:     input
 
-      if MutationObserver
+      if window.MutationObserver
         observer = new MutationObserver @load.bind @
         observer.observe @nodes.challenge,
           childList: true