--- a/browser/base/content/blockedSite.xhtml
+++ b/browser/base/content/blockedSite.xhtml
@@ -129,18 +129,22 @@
           el = document.getElementById("errorTitleText_harmful");
           el.remove();
           el = document.getElementById("errorShortDescText_harmful");
           el.remove();
           el = document.getElementById("errorLongDescText_harmful");
           el.remove();
         }
 
-        // Set sitename
-        document.getElementById(error + "_sitename").textContent = getHostString();
+        // Set sitename if necessary.
+        let sitenameElem = document.getElementById(error + "_sitename");
+        if (sitenameElem) {
+          sitenameElem.textContent = getHostString();
+        }
+
         document.title = document.getElementById("errorTitleText_" + error)
                                  .innerHTML;
 
         if (!getOverride()) {
           var btn = document.getElementById("ignoreWarningButton");
           if (btn) {
             btn.remove();
           }