diff --git a/src/site/SW.yotsuba.coffee b/src/site/SW.yotsuba.coffee
index f2035638f..8ad7dda4e 100644
--- a/src/site/SW.yotsuba.coffee
+++ b/src/site/SW.yotsuba.coffee
@@ -62,6 +62,7 @@ SW.yotsuba =
   isThisPageLegit: ->
     # not 404 error page or similar.
     location.hostname is 'boards.4chan.org' and
+    d.doctype and
     !$('link[href*="favicon-status.ico"]', d.head) and
     d.title not in ['4chan - Temporarily Offline', '4chan - Error', '504 Gateway Time-out', 'MathJax Equation Source']