diff --git a/autospec/config.py b/autospec/config.py
index 798a964..220663b 100644
--- a/autospec/config.py
+++ b/autospec/config.py
@@ -53,7 +53,7 @@ autoreconf = False
 license_fetch = None
 license_show = None
 git_uri = None
-os_packages = None
+os_packages = set()
 config_file = None
 old_version = None
 old_patches = list()