diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 05ea48a..e966cb6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,6 +16,4 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: cachix/install-nix-action@v12
-        with:
-          nix_path: nixpkgs=channel:nixos-20.09
       - run: ./test.sh
diff --git a/README.md b/README.md
index 17292b5..10cc379 100644
--- a/README.md
+++ b/README.md
@@ -16,14 +16,14 @@ You might choose to simply copy the derivations from this repository, or you can
 
 ```
 > nix repl
-nix-repl> pkgs = import <nixpkgs> {}
+nix-repl> pkgs = import ./nixpkgs.nix {}
 
 nix-repl> drvs = import (pkgs.fetchFromGitHub {
   owner = "justinwoo";
   repo = "easy-dhall-nix";
   rev = # some REV
   sha256 = # some SHA
-}) {}
+}) { inherit pkgs; }
 
 nix-repl> drvs.dhall-simple
 «derivation /nix/store/qz29jbplpmlvsbmq05084dh1fbs8sl0h-dhall-simple.drv»
diff --git a/default.nix b/default.nix
index 34cbe4c..85c9cbf 100644
--- a/default.nix
+++ b/default.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 {
   dhall-simple = import ./dhall-simple.nix {
diff --git a/dhall-bash-simple.nix b/dhall-bash-simple.nix
index 2704763..9ff2296 100644
--- a/dhall-bash-simple.nix
+++ b/dhall-bash-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-bash-simple";
diff --git a/dhall-docs-simple.nix b/dhall-docs-simple.nix
index 3b581f4..ac54212 100644
--- a/dhall-docs-simple.nix
+++ b/dhall-docs-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-docs-simple";
diff --git a/dhall-json-simple.nix b/dhall-json-simple.nix
index cceb52a..cfdf524 100644
--- a/dhall-json-simple.nix
+++ b/dhall-json-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-json-simple";
diff --git a/dhall-lsp-simple.nix b/dhall-lsp-simple.nix
index 9c12846..bf15163 100644
--- a/dhall-lsp-simple.nix
+++ b/dhall-lsp-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-lsp-simple";
diff --git a/dhall-nix-simple.nix b/dhall-nix-simple.nix
index 3c3c5c5..21fa5a0 100644
--- a/dhall-nix-simple.nix
+++ b/dhall-nix-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-nix-simple";
diff --git a/dhall-simple.nix b/dhall-simple.nix
index f37bef5..9d419ea 100644
--- a/dhall-simple.nix
+++ b/dhall-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-simple";
diff --git a/dhall-yaml-simple.nix b/dhall-yaml-simple.nix
index 1c0a8cd..f672652 100644
--- a/dhall-yaml-simple.nix
+++ b/dhall-yaml-simple.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 import ./build.nix { inherit pkgs; release = import ./release.nix; } {
   simpleName = "dhall-yaml-simple";
diff --git a/fetch.py b/fetch.py
index 974aeb0..3fdef9c 100755
--- a/fetch.py
+++ b/fetch.py
@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -i python3 -p python3 curl nix
+#!nix-shell -I nixpkgs=./nixpkgs.nix -i python3 -p python3 curl nix
 
 import json
 import re
@@ -11,6 +11,29 @@ def curl_latest_release():
     url = "https://api.github.com/repos/dhall-lang/dhall-haskell/releases/latest"
     return json.loads(sub.check_output(["curl", url]))
 
+# fetch the latest nixos unstable version
+def curl_latest_nixos_unstable():
+    url = "https://nixos.org/channels/nixos-unstable/git-revision"
+    return sub.check_output(["curl", "--location",  url]).strip().decode()
+
+def update_nixpkgs(lockfile_path, new_hash):
+    # --unpack produces the hash required by `builtins.fetchTarball`
+    github_archive = "https://github.com/NixOS/nixpkgs/archive/{}.tar.gz".format(new_hash)
+    hash = sub.check_output([
+        "nix-prefetch-url", '--unpack', github_archive
+    ]).strip().decode()
+    date = sub.check_output([
+        "date", "--iso-8601=minutes"
+    ]).strip().decode()
+    with open(lockfile_path, 'w') as f:
+        json.dump({
+            "comment": "autogenerated by fetch.py",
+            "url": github_archive,
+            "sha256": hash,
+            "date": date
+        }, f, indent=2)
+
+
 # call nix-prefetch-url on each asset to get their hashes
 def prefetch_binaries(release):
     res = []
@@ -39,11 +62,17 @@ def postprocess(fetched):
 
 
 if __name__ == "__main__":
+    print("updating nixpkgs to latest unstable", file=sys.stderr)
+    nixos_hash = curl_latest_nixos_unstable()
+    update_nixpkgs("./nixpkgs.json", nixos_hash)
+
     release = curl_latest_release()
     version = release['tag_name']
+
     print("updating to release {}".format(version), file=sys.stderr)
     fetched = prefetch_binaries(release)
     res = postprocess(fetched)
+
     print("writing ./release.json", file=sys.stderr)
     with open("./release.json", mode='w') as f:
         json.dump(res, f, indent=2)
diff --git a/nixpkgs.json b/nixpkgs.json
new file mode 100644
index 0000000..f1f1fa5
--- /dev/null
+++ b/nixpkgs.json
@@ -0,0 +1,6 @@
+{
+  "comment": "autogenerated by fetch.py",
+  "url": "https://github.com/NixOS/nixpkgs/archive/24eb3f87fc610f18de7076aee7c5a84ac5591e3e.tar.gz",
+  "sha256": "1ca14hhbinnz1ylbqnhwinjdbm6nn859j4gmyamg2kr7jl6611s0",
+  "date": "2020-11-30T23:28+01:00"
+}
\ No newline at end of file
diff --git a/nixpkgs.nix b/nixpkgs.nix
index 219a3cf..8a9556a 100644
--- a/nixpkgs.nix
+++ b/nixpkgs.nix
@@ -1,6 +1,6 @@
-import (
-  builtins.fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/a7bf8161fa834a602278c15fcbdd955656b3aed8.tar.gz";
-    sha256 = "0lzd35niw0j8qy4mhglvqwv5qwyvdb9mhv8hpmn73ym57r92i4p1";
-  }
-)
+import (fetchTarball {
+  inherit (builtins.fromJSON (builtins.readFile ./nixpkgs.json))
+    url
+    sha256
+    ;
+})
diff --git a/test.nix b/test.nix
index b408f6f..2c94b25 100644
--- a/test.nix
+++ b/test.nix
@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import ./nixpkgs.nix {} }:
 
 let
 
diff --git a/test.sh b/test.sh
index 6243540..c081b68 100755
--- a/test.sh
+++ b/test.sh
@@ -1,4 +1,8 @@
 #!/usr/bin/env sh
 set -e
-test_script="$(nix-build ./test.nix)"
-exec "$test_script"
+tmp="$(mktemp -d)"
+trap "rm -r $tmp" EXIT
+nix-build \
+    --out-link "$tmp"/result \
+    ./test.nix
+"$tmp/result"