diff --git a/check-fmt b/check-fmt
index e3a5587..1fe212b 100755
--- a/check-fmt
+++ b/check-fmt
@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#!nix-shell ./shell.nix -i bash
+#!nix-shell ./check-fmt.nix -i bash
 #
 # Just because the nixpkgs-fmt error message is not super readable. Used by
 # CI.
diff --git a/check-fmt.nix b/check-fmt.nix
new file mode 100644
index 0000000..ab5f223
--- /dev/null
+++ b/check-fmt.nix
@@ -0,0 +1,7 @@
+let
+  sources = import ./nix/sources.nix;
+  pkgs = import sources.nixpkgs { };
+in
+pkgs.mkShell {
+  nativeBuildInputs = [ pkgs.nixpkgs-fmt ];
+}