diff --git a/src/calibre/ebooks/conversion/plugins/epub_output.py b/src/calibre/ebooks/conversion/plugins/epub_output.py
index a7f20f7e3b..bcf93728e4 100644
--- a/src/calibre/ebooks/conversion/plugins/epub_output.py
+++ b/src/calibre/ebooks/conversion/plugins/epub_output.py
@@ -299,7 +299,7 @@ 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)
-        previous_nav = (nav_href, existing_nav) if existing_nav and nav_href else None
+        previous_nav = (nav_href, existing_nav) if existing_nav is not None 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()