diff --git a/.circleci/config.yml b/.circleci/config.yml
index 0bbf991..4583ed2 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -12,6 +12,7 @@ jobs:
             sudo chown circleci /nix
             bash <(curl https://nixos.org/nix/install)
             echo '. /home/circleci/.nix-profile/etc/profile.d/nix.sh' >> $BASH_ENV
+            sudo mkdir -p /etc/nix && echo "sandbox = true" | sudo tee -a /etc/nix/nix.conf
       - checkout
       - restore_cache:
             # Match on the latest cache for this branch
@@ -35,8 +36,7 @@ jobs:
 
       - run:
           name: Nix build
-          command: |
-            nix-build --no-out-link --option sandbox true
+          command: ./script/test
 
       - run:
           name: Create cache
diff --git a/default.nix b/default.nix
index 26028ad..765e32e 100644
--- a/default.nix
+++ b/default.nix
@@ -1,2 +1,2 @@
-with { pkgs = import ./nix; };
-{ inherit (pkgs) niv readme ; }
+with { pkgs = import ./nix {}; };
+{ inherit (pkgs) niv readme readme-test ; }
diff --git a/nix/default.nix b/nix/default.nix
index f5efbae..b6c5d55 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,3 +1,3 @@
-with { sources = import ./sources.nix; };
+{ sources ? import ./sources.nix }:
 import sources.nixpkgs
   { overlays = import ./overlay.nix { inherit sources; } ; config = {}; }
diff --git a/nix/overlay.nix b/nix/overlay.nix
index 09f8dbc..aba0b87 100644
--- a/nix/overlay.nix
+++ b/nix/overlay.nix
@@ -16,7 +16,7 @@
 
   # README generation
   (self: super:
-    { readme = self.writeText "README.md"
+    rec { readme = self.writeText "README.md"
         (with
           { template = builtins.readFile ../README.tpl.md;
             niv_help = builtins.readFile
@@ -46,6 +46,8 @@
           [ niv_help niv_add_help niv_update_help niv_drop_help ]
           template
         );
+      readme-test = self.runCommand "README-test" {}
+        "diff ${../README.md} ${readme} && echo dummy > $out";
     }
   )
 ]
diff --git a/nix/sources.nix b/nix/sources.nix
index 8135bd5..2ae3408 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -10,10 +10,13 @@ with
         builtins.fetchTarball { inherit url; }
       else
         builtins.fetchTarball { inherit url sha256; };
+  mapAttrs = builtins.mapAttrs or
+    (f: set: with builtins;
+      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
 };
 
 # NOTE: spec must _not_ have an "outPath" attribute
-builtins.mapAttrs (_: spec:
+mapAttrs (_: spec:
   if builtins.hasAttr "outPath" spec
   then abort
     "The values in versions.json should not have an 'outPath' attribute"
diff --git a/script/test b/script/test
new file mode 100755
index 0000000..4c73862
--- /dev/null
+++ b/script/test
@@ -0,0 +1,11 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+#!nix-shell -I nixpkgs=./nix
+#!nix-shell -p nix
+#!nix-shell --pure
+
+echo "Building"
+
+nix build --no-link
+
+echo "all good"