diff --git a/src/calibre/gui2/tweak_book/file_list.py b/src/calibre/gui2/tweak_book/file_list.py
index a5716f644b..1762386469 100644
--- a/src/calibre/gui2/tweak_book/file_list.py
+++ b/src/calibre/gui2/tweak_book/file_list.py
@@ -496,7 +496,7 @@ class FileList(QTreeWidget):
 
     def show_context_menu(self, point):
         item = self.itemAt(point)
-        if item is None or item in set(itervalues(self.categories)):
+        if item is None or item in tuple(itervalues(self.categories)):
             return
         m = QMenu(self)
         sel = self.selectedItems()