diff --git a/setup.cfg b/setup.cfg
index c7858f35b788256d5b94c5b5750e12b5c8a5a75d..7ea8404d9900ece604ea87101892a9e13002feeb 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -4,6 +4,7 @@ typecheck=pytest --addopts="--mypy conservancy_beancount"
 
 [mypy]
 disallow_any_unimported = True
+disallow_untyped_defs = True
 show_error_codes = True
 strict_equality = True
 warn_redundant_casts = True