diff --git a/.travis.yml b/.travis.yml
index bef15a4..759dfc5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -42,6 +42,7 @@ language: shell
 
 .nix: &NIX
   language: nix
+  nix: 2.3.5
   script:
   - nix-build --argstr coq-url "$COQ" --extra-substituters https://coq.cachix.org --trusted-public-keys "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= coq.cachix.org-1:5QW/wwEnD+l2jvN6QRbRRsa4hBHG3QiQQ26cxu1F5tI="