diff --git a/src/calibre/ebooks/metadata/fb2.py b/src/calibre/ebooks/metadata/fb2.py
index 4271fdfaea..7fd2de3d6e 100644
--- a/src/calibre/ebooks/metadata/fb2.py
+++ b/src/calibre/ebooks/metadata/fb2.py
@@ -34,7 +34,7 @@ class Context(object):
 
     def __init__(self, root):
         try:
-            self.fb_ns = root.nsmap[root.prefix]
+            self.fb_ns = root.nsmap[root.prefix] or NAMESPACES['fb2']
         except Exception:
             self.fb_ns = NAMESPACES['fb2']
         self.namespaces = {