diff --git a/nixpkgs.nix b/nixpkgs.nix
index ac619bc4..4a20b471 100644
--- a/nixpkgs.nix
+++ b/nixpkgs.nix
@@ -1,9 +1,9 @@
 let
-  # 2020-08-31 master
-  rev = "6716867eb3e763818000eab04f378b86cadc2894";
-  sha256 = "19fnqwpq1rk4iibbgq04j9w72s4n31nlz7m26iqhqd5lh7p8sc42";
+  # https://github.com/tweag/nixpkgs/tree/update-ghchead-20201001
+  rev = "647a878e061b1b63efd2094eb9e546c2a7aa0d58";
+  sha256 = "1cfn53nls9kfh4ql8bn94zgvajm93bd390927fi345d943hnph84";
 in
 import (fetchTarball {
   inherit sha256;
-  url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+  url = "https://github.com/utdemir/nixpkgs/archive/${rev}.tar.gz";
 })
diff --git a/stack.yaml b/stack.yaml
index c41363fa..31561be2 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,6 +1,6 @@
 # See https://hub.docker.com/r/tweag/linear-types/
 resolver: lts-16.15
-compiler: ghc-8.11
+compiler: ghc-9.1
 allow-newer: true
 system-ghc: true