diff --git a/4chan_x.user.js b/4chan_x.user.js
index 1a959c2fc..0215bab6a 100644
--- a/4chan_x.user.js
+++ b/4chan_x.user.js
@@ -312,7 +312,7 @@
     },
     formData: function(arg) {
       var fd, key, val;
-      if (arg instanceof HTMLElement) {
+      if (arg instanceof HTMLFormElement) {
         fd = new FormData(arg);
       } else {
         fd = new FormData();
diff --git a/script.coffee b/script.coffee
index 6005d5a37..43fc6f1b8 100644
--- a/script.coffee
+++ b/script.coffee
@@ -266,7 +266,7 @@ $.extend $,
   id: (id) ->
     d.getElementById id
   formData: (arg) ->
-    if arg instanceof HTMLElement
+    if arg instanceof HTMLFormElement
       fd = new FormData arg
     else
       fd = new FormData()