diff --git a/src/calibre/customize/ui.py b/src/calibre/customize/ui.py
index 8f3b874f31..4d0e787040 100644
--- a/src/calibre/customize/ui.py
+++ b/src/calibre/customize/ui.py
@@ -115,6 +115,7 @@ def reread_filetype_plugins():
     global _on_postimport
     global _on_preprocess
     global _on_postprocess
+    global _on_postadd
     _on_import           = defaultdict(list)
     _on_postimport       = defaultdict(list)
     _on_preprocess       = defaultdict(list)