diff --git a/src/calibre/ebooks/oeb/transforms/jacket.py b/src/calibre/ebooks/oeb/transforms/jacket.py
index c27ceb99e8..53f450ddf2 100644
--- a/src/calibre/ebooks/oeb/transforms/jacket.py
+++ b/src/calibre/ebooks/oeb/transforms/jacket.py
@@ -273,6 +273,7 @@ def render_jacket(mi, output_profile,
                 elif dt == 'rating':
                     args[dkey] = rating_to_stars(mi.get(key), m.get('display', {}).get('allow_half_stars', False))
                 elif dt == 'comments':
+                    val = val or ''
                     display = m.get('display', {})
                     ctype = display.get('interpret_as') or 'html'
                     if ctype == 'long-text':