diff --git a/src/calibre/gui2/dialogs/metadata_single.py b/src/calibre/gui2/dialogs/metadata_single.py
index 346bf97336..de8fc516ef 100644
--- a/src/calibre/gui2/dialogs/metadata_single.py
+++ b/src/calibre/gui2/dialogs/metadata_single.py
@@ -330,6 +330,8 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
                 if not ext:
                     ext = ''
                 size = self.db.sizeof_format(row, ext)
+                if size is None:
+                    continue
                 Format(self.formats, ext, size)