diff --git a/src/calibre/ebooks/metadata/book/base.py b/src/calibre/ebooks/metadata/book/base.py
index 69a3c42f4d..be9a4675c0 100644
--- a/src/calibre/ebooks/metadata/book/base.py
+++ b/src/calibre/ebooks/metadata/book/base.py
@@ -156,8 +156,12 @@ class Metadata(object):
         the key name not the label
         '''
         if field is not None:
+            if not field.startswith('#'):
+                raise AttributeError(
+                        'Custom field name %s must begin with \'#\''%repr(field))
             if metadata is None:
                 traceback.print_stack()
+                return
             metadata = copy.deepcopy(metadata)
             if '#value#' not in metadata:
                 if metadata['datatype'] == 'text' and metadata['is_multiple']: