diff --git a/nix/sources.nix b/nix/sources.nix
index 7a9a2ae..d4ac577 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -47,6 +47,10 @@ with rec
   # annoyingly this means we have to specify them
   fetchzip = { url, sha256 }@attrs: pkgs.fetchzip attrs;
 
+  # A wrapper around pkgs.fetchurl that has inspectable arguments,
+  # annoyingly this means we have to specify them
+  fetchurl = { url, sha256 }@attrs: pkgs.fetchurl attrs;
+
   hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
   hasThisAsNixpkgsPath =
     (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
@@ -71,7 +75,7 @@ with rec
     in builtins.getAttr fetcherName {
       "tarball" = fetchzip;
       "builtin-tarball" = builtins_fetchTarball;
-      "file" = pkgs.fetchurl;
+      "file" = fetchurl;
       "builtin-url" = builtins_fetchurl;
     };
 };