diff --git a/src/calibre/ebooks/conversion/plugins/epub_output.py b/src/calibre/ebooks/conversion/plugins/epub_output.py
index 3741d3c9a4..ae164f753d 100644
--- a/src/calibre/ebooks/conversion/plugins/epub_output.py
+++ b/src/calibre/ebooks/conversion/plugins/epub_output.py
@@ -298,7 +298,8 @@ class EPUBOutput(OutputFormatPlugin):
         from calibre.ebooks.oeb.polish.upgrade import epub_2_to_3
         existing_nav = getattr(self.opts, 'epub3_nav_parsed', None)
         nav_href = getattr(self.opts, 'epub3_nav_href', None)
-        epub_2_to_3(container, self.log.info, previous_nav=(nav_href, existing_nav))
+        previous_nav = (existing_nav, nav_href) if existing_nav and nav_href else None
+        epub_2_to_3(container, self.log.info, previous_nav=previous_nav)
         fix_conversion_titlepage_links_in_nav(container)
         container.commit()
         os.remove(f.name)