diff --git a/pyproject.toml b/pyproject.toml
index fd22aec..c5ff8af 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,6 +15,7 @@ dependencies = [
     "aiofiles>=23.0",
     "websockets>=11.0",
     "python-pam>=1.8.4",
+    "six>=1.16.0",              # undeclared dep of python-pam (needed on macOS clean envs)
     "itsdangerous>=2.1.0",
     "python-multipart>=0.0.9",
 ]