diff --git a/tox.ini b/tox.ini
index 0a28af1..e72eb58 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,7 +8,7 @@
     pretend
 commands =
     coverage run --source=cryptography/,tests/ -m pytest
-    coverage report -m
+    coverage report -m --fail-under 100
 
 [testenv:docs]
 deps = sphinx