diff --git a/nix/sources.json b/nix/sources.json
index 9b76c6bd..6012b1c0 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -29,10 +29,10 @@
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "772406c2a4e22a85620854056a4cd02856fa10f0",
-        "sha256": "0kdckm2wlxdpaslwhlvi6mlbl33p3wcn5677wwmm1anqz7287pap",
+        "rev": "842f900e73c7ce985218cc4f455e34d1d56475c1",
+        "sha256": "0ygg3zkvcxy7yhl00d5l07sfpnbld4321qpcw8z68mkc3ncyv3ws",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/772406c2a4e22a85620854056a4cd02856fa10f0.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/842f900e73c7ce985218cc4f455e34d1d56475c1.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/nix/sources.nix b/nix/sources.nix
index b796fffa..1938409d 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -98,7 +98,10 @@ let
       saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
       ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
     in
-      if ersatz == "" then drv else ersatz;
+      if ersatz == "" then drv else
+        # this turns the string into an actual Nix path (for both absolute and
+        # relative paths)
+        if builtins.substring 0 1 ersatz == "/" then /. + ersatz else /. + builtins.getEnv "PWD" + "/${ersatz}";
 
   # Ports of functions for older nix versions
 
diff --git a/shell.nix b/shell.nix
index c701ff5c..47e258ec 100644
--- a/shell.nix
+++ b/shell.nix
@@ -26,7 +26,7 @@ pkgs.mkShell {
     pythonEnv
     fake-lsb-release
     mypy
-    haskell.compiler.ghc8103
+    haskell.compiler.ghc8104
     haskellPackages.cabal-install
     haskellPackages.tasty-discover
     haskellPackages.cabal-fmt # For formatting .cabal files, example
diff --git a/src/ldfi/default.nix b/src/ldfi/default.nix
index 0c8b833f..cf42e378 100644
--- a/src/ldfi/default.nix
+++ b/src/ldfi/default.nix
@@ -1,5 +1,5 @@
 { sources ? import ./../../nix/sources.nix
-, compiler ? "ghc8103"
+, compiler ? "ghc8104"
 }:
 let
   inherit (import sources.gitignore { }) gitignoreSource;
diff --git a/src/ltl/default.nix b/src/ltl/default.nix
index 541be4fa..04b4f7c6 100644
--- a/src/ltl/default.nix
+++ b/src/ltl/default.nix
@@ -1,5 +1,5 @@
 { sources ? import ./../../nix/sources.nix
-, compiler ? "ghc8103"
+, compiler ? "ghc8104"
 }:
 let
   inherit (import sources.gitignore { }) gitignoreSource;