diff -r 723261fb23fb -r ee0c833d1969 src/cm/media/js/client/c_edit_form.js
--- a/src/cm/media/js/client/c_edit_form.js	Thu Oct 10 11:16:05 2013 +0200
+++ b/src/cm/media/js/client/c_edit_form.js	Tue Oct 15 12:12:50 2013 +0200
@@ -61,8 +61,10 @@
   // => hides these inputs for a reply.
   if (comment.reply_to_id != null) {
     CY.get("#"+gEdit['ids']['changeScopeInputId']).addClass('displaynone')
-    CY.get("#"+gEdit['ids']['categoryInputId']).addClass('displaynone')
-    CY.get("#"+gEdit['ids']['categoryInputId']).ancestor().addClass('displaynone')
+    if (CY.get("#"+gEdit['ids']['categoryInputId'])) {
+      CY.get("#"+gEdit['ids']['categoryInputId']).addClass('displaynone')
+      CY.get("#"+gEdit['ids']['categoryInputId']).ancestor().addClass('displaynone')
+    }
   }
   changeScopeFormClick() ; // to adapt