diff --git a/src/calibre/gui2/tools.py b/src/calibre/gui2/tools.py
index 7e1655b425..5670031e68 100644
--- a/src/calibre/gui2/tools.py
+++ b/src/calibre/gui2/tools.py
@@ -198,10 +198,11 @@ def convert_existing(parent, db, book_ids, output_format):
             already_converted_ids.append(book_id)
             already_converted_titles.append(db.get_metadata(book_id, True).title)
 
-    if not question_dialog(parent, _('Convert existing'),
-            _('The following books have already been converted to %s format. '
-               'Do you wish to reconvert them?') % output_format,
-            '\n'.join(already_converted_titles)):
-        book_ids = [x for x in book_ids if x not in already_converted_ids]
+    if already_converted_ids:
+        if not question_dialog(parent, _('Convert existing'),
+                _('The following books have already been converted to %s format. '
+                   'Do you wish to reconvert them?') % output_format,
+                '\n'.join(already_converted_titles)):
+            book_ids = [x for x in book_ids if x not in already_converted_ids]
 
     return book_ids