diff --git a/src/calibre/utils/browser.py b/src/calibre/utils/browser.py
index 75799aba5a..2f77ede6b3 100644
--- a/src/calibre/utils/browser.py
+++ b/src/calibre/utils/browser.py
@@ -93,4 +93,6 @@ if __name__ == '__main__':
     assert orig._ua_handlers.keys() == clone._ua_handlers.keys()
     assert orig._ua_handlers['_cookies'].cookiejar is \
             clone._ua_handlers['_cookies'].cookiejar
+    assert orig.addheaders == clone.addheaders
+