diff --git a/src/calibre/ebooks/lit/oeb.py b/src/calibre/ebooks/lit/oeb.py
index d9fc73f249..fb1aa14cd6 100644
--- a/src/calibre/ebooks/lit/oeb.py
+++ b/src/calibre/ebooks/lit/oeb.py
@@ -312,7 +312,7 @@ class Manifest(object):
             yield id, item
     
     def __contains__(self, key):
-        return id in self.ids
+        return key in self.ids
 
     def to_opf1(self, parent=None):
         elem = element(parent, 'manifest')