diff --git a/src/calibre/ebooks/metadata/opf3.py b/src/calibre/ebooks/metadata/opf3.py
index 2cf3ff746e..b4619fbb60 100644
--- a/src/calibre/ebooks/metadata/opf3.py
+++ b/src/calibre/ebooks/metadata/opf3.py
@@ -1118,7 +1118,8 @@ def apply_metadata(root, mi, cover_prefix='', cover_data=None, apply_null=False,
     if ok('rating') and mi.rating is not None and float(mi.rating) > 0.1:
         set_rating(root, prefixes, refines, mi.rating)
     if ok('series'):
-        set_series(root, prefixes, refines, mi.series, mi.series_index or 1)
+        sidx = mi.series_index if isinstance(mi.series_index, (int, float)) else 1.0
+        set_series(root, prefixes, refines, mi.series, sidx)
     if ok('link_maps'):
         set_link_maps(root, prefixes, refines, getattr(mi, 'link_maps', None))
     if ok('user_categories'):