diff --git a/packages/child/index.js b/packages/child/index.js
index 6499793c0..8ddb53466 100644
--- a/packages/child/index.js
+++ b/packages/child/index.js
@@ -198,7 +198,7 @@ function iframeResizerChild() {
   }
 
   function checkQuirksMode() {
-    if (document.compatMode === 'BackCompat') return
+    if (document.compatMode !== 'BackCompat') return
 
     advise(
       `<rb>Quirks Mode Detected</>