diff --git a/nix/default.nix b/nix/default.nix
index 8d53ae31d89aeacc971f5f02b0acbef5fe477e1e..9605b5b0c6e1a7ab470edbce26fa13aee556cd2b 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,7 +1,12 @@
 { pkgs ? import <nixpkgs> {} }:
 
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) { inherit pkgs; };
+  # Pin a particular version of haskell.nix.  The particular version isn't
+  # special.  It's just recent at the time this expression was written and it
+  # is known to work with PaymentServer.  It could be bumped if necessary but
+  # this would probably only happen as a result of bumping the resolver in
+  # stack.yaml.
+  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/0cb32e695d7014908fb01fd7e3d225ea33dbdc98.tar.gz) { inherit pkgs; };
 
   pkgSet = haskell.mkStackPkgSet {
     stack-pkgs = import ./pkgs.nix;