diff --git a/libmat2/abstract.py b/libmat2/abstract.py
index 9cb9320..b7ccae9 100644
--- a/libmat2/abstract.py
+++ b/libmat2/abstract.py
@@ -2,6 +2,8 @@ import abc
 import os
 from typing import Set
 
+assert Set  # make pyflakes happy
+
 
 class AbstractParser(abc.ABC):
     meta_list = set()  # type: Set[str]