diff --git a/src/calibre/ebooks/metadata/book/base.py b/src/calibre/ebooks/metadata/book/base.py
index 3be37d922e..1681789528 100644
--- a/src/calibre/ebooks/metadata/book/base.py
+++ b/src/calibre/ebooks/metadata/book/base.py
@@ -529,8 +529,8 @@ class Metadata(object):
                             for t in st.intersection(ot):
                                 sidx = lstags.index(t)
                                 oidx = lotags.index(t)
-                                self_tags[sidx] = other.tags[oidx]
-                            self_tags += [t for t in other.tags if t.lower() in ot-st]
+                                self_tags[sidx] = other_tags[oidx]
+                            self_tags += [t for t in other_tags if t.lower() in ot-st]
                             setattr(self, x, self_tags)
 
             my_comments = getattr(self, 'comments', '')