diff --git a/src/calibre/devices/interface.py b/src/calibre/devices/interface.py
index 10feeb45e7..d6159b9613 100644
--- a/src/calibre/devices/interface.py
+++ b/src/calibre/devices/interface.py
@@ -77,7 +77,7 @@ class DevicePlugin(Plugin):
     OPEN_FEEDBACK_MESSAGE = None
 
     #: Set of extensions that are "virtual books" on the device
-    #: and therefore cannot be viewed/saved/added to library
+    #: and therefore cannot be viewed/saved/added to library.
     #: For example: ``frozenset(['kobo'])``
     VIRTUAL_BOOK_EXTENSIONS = frozenset([])