diff --git a/tox.ini b/tox.ini
index 69f8159..d5228c1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,6 +15,7 @@ setenv =
 passenv =
     HOME
     SETUPTOOLS_*
+    GYPSUM_CACHE_DIR
 extras =
     testing
 commands =