diff --git a/src/calibre/gui2/viewer/highlights.py b/src/calibre/gui2/viewer/highlights.py
index c52049b272..dc23261c97 100644
--- a/src/calibre/gui2/viewer/highlights.py
+++ b/src/calibre/gui2/viewer/highlights.py
@@ -277,7 +277,8 @@ class Highlights(QTreeWidget):
 
         def cfi_key(h):
             cfi = h.get('start_cfi')
-            return (h.get('spine_index') or defval[0], cfi_sort_key(cfi)) if cfi else defval
+            si = h.get('spine_index')
+            return (defval[0] if si is None else si, cfi_sort_key(cfi)) if cfi else defval
 
         return sorted(highlights, key=cfi_key)