diff --git a/setup.cfg b/setup.cfg
index 85743dbf49..e11048c6c9 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -17,3 +17,4 @@ line_length = 85
 multi_line_output = 5
 known_future_library = __python__
 known_standard_library = aes,elementmaker,encodings
+known_first_party = calibre_extensions