diff --git a/src/calibre/ebooks/metadata/book/base.py b/src/calibre/ebooks/metadata/book/base.py
index ab0b9f649a..f5c5d9adaa 100644
--- a/src/calibre/ebooks/metadata/book/base.py
+++ b/src/calibre/ebooks/metadata/book/base.py
@@ -185,10 +185,11 @@ class Metadata(object):
         return key in object.__getattribute__(self, '_data')
 
     def deepcopy(self):
-        ''' Do not use this method unless you know what you are doing, if you want to create a simple clone of
-        this object, use :meth:`deepcopy_metadata` instead. '''
+        ''' Do not use this method unless you know what you are doing, if you
+        want to create a simple clone of this object, use :meth:`deepcopy_metadata`
+        instead. '''
         m = Metadata(None)
-        m.__dict__ = copy.deepcopy(self.__dict__)
+        object.__setattr__(m, '__dict__', copy.deepcopy(self.__dict__))
         object.__setattr__(m, '_data', copy.deepcopy(object.__getattribute__(self, '_data')))
         return m