diff --git a/src/calibre/ebooks/metadata/extz.py b/src/calibre/ebooks/metadata/extz.py
index 21c10278e1..1bda263015 100644
--- a/src/calibre/ebooks/metadata/extz.py
+++ b/src/calibre/ebooks/metadata/extz.py
@@ -70,7 +70,8 @@ def set_metadata(stream, mi):
     # Update the metadata.
     old_mi = opf.to_book_metadata()
     old_mi.smart_update(mi)
-    newopf = StringIO(metadata_to_opf(old_mi))
+    opf.smart_update(metadata_to_opf(old_mi))
+    newopf = StringIO(opf.render())
     safe_replace(stream, opf_path, newopf, extra_replacements=replacements, add_missing=True)
 
     # Cleanup temporary files.