diff --git a/default.nix b/default.nix
index c7f1d936a..ed7f9637f 100644
--- a/default.nix
+++ b/default.nix
@@ -35,15 +35,15 @@ mixRelease rec {
         fast_html = prev.fast_html.override {
           nativeBuildInputs = [ cmake ];
         };
-        ex_cldr = prev.ex_cldr.overrideAttrs (old: rec {
-          version = "2.37.5";
+        ex_cldr = prev.ex_cldr.overrideAttrs (old: {
           # We have to use the GitHub sources, as it otherwise tries to download
           # the locales at build time.
           src = fetchFromGitHub {
             owner = "elixir-cldr";
             repo = "cldr";
-            rev = "v${version}";
-            sha256 = "sha256-T5Qvuo+xPwpgBsqHNZYnTCA4loToeBn1LKTMsDcCdYs=";
+            rev = "v${old.version}";
+            sha256 = assert old.version == "2.37.5";
+              "sha256-T5Qvuo+xPwpgBsqHNZYnTCA4loToeBn1LKTMsDcCdYs=";
           };
           postInstall = ''
             cp $src/priv/cldr/locales/* $out/lib/erlang/lib/ex_cldr-${old.version}/priv/cldr/locales/