diff --git a/pkgs/build-support/rust/fetchcargo.nix b/pkgs/build-support/rust/fetchcargo.nix
index 1a40a362d9b..eb51e5c4ff9 100644
--- a/pkgs/build-support/rust/fetchcargo.nix
+++ b/pkgs/build-support/rust/fetchcargo.nix
@@ -4,6 +4,13 @@ let cargo-vendor-normalise = stdenv.mkDerivation {
   src = ./cargo-vendor-normalise.py;
   unpackPhase = ":";
   installPhase = "install -D $src $out/bin/cargo-vendor-normalise";
+  doInstallCheck = true;
+  installCheckPhase = ''
+    # check that ./fetchcargo-default-config.toml is a fix point
+    reference=${./fetchcargo-default-config.toml}
+    < $reference $out/bin/cargo-vendor-normalise > test;
+    cmp test $reference
+  '';
   buildInputs = [ (python3.withPackages(ps: [ ps.toml ])) ];
   preferLocalBuild = true;
 };