diff --git a/nix/ci.nix b/nix/ci.nix
index b2b13a1..58e62e4 100644
--- a/nix/ci.nix
+++ b/nix/ci.nix
@@ -7,7 +7,6 @@ in
 dimension "Nixpkgs version" {
     "nixos-20_09" = {
       nixpkgsSource = "nixos-20.09";
-      isReferenceNixpkgs = true;
       enableDoc = true;
       dockerSupportsSystemd = true;
       nixosHasPodmanDockerSocket = false;
@@ -18,6 +17,7 @@ dimension "Nixpkgs version" {
     };
     "nixos-unstable" = {
       nixpkgsSource = "nixos-unstable";
+      isReferenceNixpkgs = true; # match ./default.nix
       enableDoc = true;
     };
   } (
diff --git a/nix/default.nix b/nix/default.nix
index 1bafe0e..5cc58ac 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,5 +1,5 @@
 { sources ? import ./sources.nix
-, nixpkgsName ? "nixos-unstable"
+, nixpkgsName ? "nixos-unstable" # match ./ci.nix isReferenceNixpkgs
 , nixpkgsSrc ? sources.${nixpkgsName}
 , system ? builtins.currentSystem
 , dockerSupportsSystemd ? false