diff --git a/default.nix b/default.nix
index 68fc3b2..ec64995 100644
--- a/default.nix
+++ b/default.nix
@@ -6,29 +6,36 @@ let
     let newSrc = lib.cleanSourceWith { filter = f: t: true; src = ./.; };
     in (builtins.functionArgs lib.cleanSourceWith) ? name || newSrc ? name;
 
-in
-{
-  inherit (find-files) gitignoreFilter;
-
   gitignoreSource =
     if newCleanSourceWith
     then
-      path:
-        let
-          origPath = path.origPath or path;
-        in
-        lib.cleanSourceWith {
-          name = "source";
-          filter = find-files.gitignoreFilter origPath;
-          src = path;
-        }
+      path: gitignoreSourceWith { inherit path; }
     else
       path:
         if path ? _isLibCleanSourceWith
         then builtins.abort "Sorry, please update your Nixpkgs to 19.09 or master if you want to use gitignoreSource on cleanSourceWith"
-        else builtins.path {
+        else lib.warn "You are using gitignore.nix with an old version of Nixpkgs that is not supported." (builtins.path {
           name = "source";
           filter = find-files.gitignoreFilter path;
           inherit path;
-        };
+        });
+
+  gitignoreSourceWith = { path }:
+    lib.cleanSourceWith {
+      name = "source";
+      filter = find-files.gitignoreFilterWith { basePath = path.origPath or path; };
+      src = path;
+    };
+
+in
+{
+  inherit (find-files)
+    gitignoreFilter
+    gitignoreFilterWith
+    ;
+  inherit
+    gitignoreSource
+    gitignoreSourceWith
+    ;
+
 }
diff --git a/find-files.nix b/find-files.nix
index ceccc65..09018db 100644
--- a/find-files.nix
+++ b/find-files.nix
@@ -18,6 +18,9 @@ rec {
   # TODO: write test for trailing slash (matches dir only)
 
   gitignoreFilter = basePath:
+    gitignoreFilterWith { inherit basePath; };
+
+  gitignoreFilterWith = { basePath }:
     let
       patternsBelowP = findPatternsTree basePath;
       basePathStr = toString basePath;
diff --git a/nix/ci.nix b/nix/ci.nix
index 70de687..c574b1a 100644
--- a/nix/ci.nix
+++ b/nix/ci.nix
@@ -7,6 +7,7 @@ let
   dimension "Nixpkgs" {
     "nixpkgs-19_03" = sources."nixos-19.03";
     "nixpkgs-19_09" = sources."nixos-19.09";
+    "nixpkgs-21_05" = sources."nixos-21.05";
   } (_key: nixpkgs:
     import ../tests/default.nix { pkgs = import ./default.nix { inherit nixpkgs; }; }
   )
diff --git a/nix/sources.json b/nix/sources.json
index 6c2cc8f..56cbf2a 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -35,6 +35,18 @@
         "url": "https://github.com/NixOS/nixpkgs-channels/archive/e6b068cd952e4640d416c2ccd62587d8a35cd774.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "nixos-21.05": {
+        "branch": "nixos-21.05",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "3b1789322fcbcb5cf51228d732752714f1bf77da",
+        "sha256": "0m6gcl2pzvicwbflsvmccdyf03ki1zl3d9dl8rn8hj1gdgssj6vr",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/3b1789322fcbcb5cf51228d732752714f1bf77da.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "nixpkgs": {
         "branch": "nixos-19.03",
         "description": "Nixpkgs/NixOS branches that track the Nixpkgs/NixOS channels",