diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index a2ae96ae27..ba8aeecdcf 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -1549,7 +1549,7 @@ class Boss(QObject):
             container = current_container()
             for name, (path, mt) in iteritems(name_map):
                 with lopen(path, 'rb') as f:
-                    container.add_file(name, f.read(), media_type=mt)
+                    container.add_file(name, f.read(), media_type=mt, modify_name_if_needed=True)
             self.apply_container_update_to_gui()
 
     def export_file(self, name, path):