diff --git a/src/calibre/ebooks/conversion/plumber.py b/src/calibre/ebooks/conversion/plumber.py
index 59d7a0ed2a..a4708d398c 100644
--- a/src/calibre/ebooks/conversion/plumber.py
+++ b/src/calibre/ebooks/conversion/plumber.py
@@ -532,7 +532,7 @@ def __init__(self, input, output, log, report_progress=DummyReporter(),
            'horizontal rules.')),
            
 OptionRecommendation(name='replace_scene_breaks',
-    recommended_value=None, level=OptionRecommendation.LOW,
+    recommended_value='', level=OptionRecommendation.LOW,
     help=_('Replace scene breaks with the specified text.')),
 
 OptionRecommendation(name='dehyphenate',