diff --git a/src/calibre/ebooks/metadata/__init__.py b/src/calibre/ebooks/metadata/__init__.py
index 05b624f811..856133d1ce 100644
--- a/src/calibre/ebooks/metadata/__init__.py
+++ b/src/calibre/ebooks/metadata/__init__.py
@@ -21,7 +21,10 @@ def string_to_authors(raw):
     return authors
 
 def authors_to_string(authors):
-    return ' & '.join([a.replace('&', '&&') for a in authors])
+    if authors is not None:
+        return ' & '.join([a.replace('&', '&&') for a in authors])
+    else:
+        return ''
 
 def author_to_author_sort(author):
     tokens = author.split()