diff --git a/nixos/lib/testing.nix b/nixos/lib/testing.nix
index e68563ef48d2..a13e76a69560 100644
--- a/nixos/lib/testing.nix
+++ b/nixos/lib/testing.nix
@@ -3,7 +3,7 @@
   # Use a minimal kernel?
 , minimal ? false
   # Ignored
-, config ? null
+, config ? {}
   # Modules to add to each VM
 , extraConfigurations ? [] }: