diff --git a/nix/sources.json b/nix/sources.json
index a1ff3f1..037f524 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -12,15 +12,15 @@
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
-        "branch": "nixos-unstable",
+        "branch": "nixpkgs-unstable",
         "description": "Nix Packages collection",
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "266dc8c3d052f549826ba246d06787a219533b8f",
-        "sha256": "09ydqx2lznixmw8z4cfz1j3k137mh8n3cdpygwqymknhfdjq7lg4",
+        "rev": "efee454783c5c14ae78687439077c1d3f0544d97",
+        "sha256": "1qk4g8rav2mkbd6y2zr1pi3pxs4rwlkpr8xk51m0p26khprxvjaf",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/266dc8c3d052f549826ba246d06787a219533b8f.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/efee454783c5c14ae78687439077c1d3f0544d97.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/nix/sources.nix b/nix/sources.nix
index b796fff..1938409 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