diff --git a/nix/sources.nix b/nix/sources.nix
index dc040f8..7a9a2ae 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -21,9 +21,8 @@ with rec
         add a package called "nixpkgs" to your sources.json.
     '';
 
+  # fetchTarball version that is compatible between all the versions of Nix
   builtins_fetchTarball =
-      # fetchTarball version that is compatible between all the versions of
-      # Nix
       { url, sha256 }@attrs:
       let
         inherit (builtins) lessThan nixVersion fetchTarball;
@@ -33,6 +32,21 @@ with rec
         else
           fetchTarball attrs;
 
+  # fetchurl version that is compatible between all the versions of Nix
+  builtins_fetchurl =
+      { url, sha256 }@attrs:
+      let
+        inherit (builtins) lessThan nixVersion fetchurl;
+      in
+        if lessThan nixVersion "1.12" then
+          fetchurl { inherit url; }
+        else
+          fetchurl attrs;
+
+  # A wrapper around pkgs.fetchzip that has inspectable arguments,
+  # annoyingly this means we have to specify them
+  fetchzip = { url, sha256 }@attrs: pkgs.fetchzip attrs;
+
   hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
   hasThisAsNixpkgsPath =
     (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
@@ -43,14 +57,22 @@ with rec
     (f: set: with builtins;
       listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
 
+  # borrowed from nixpkgs
+  functionArgs = f: f.__functionArgs or (builtins.functionArgs f);
+  callFunctionWith = autoArgs: f: args:
+    let auto = builtins.intersectAttrs (functionArgs f) autoArgs;
+    in f (auto // args);
+
   getFetcher = spec:
     let fetcherName =
       if builtins.hasAttr "type" spec
       then builtins.getAttr "type" spec
-      else "tarball";
+      else "builtin-tarball";
     in builtins.getAttr fetcherName {
-      "tarball" = pkgs.fetchzip;
+      "tarball" = fetchzip;
+      "builtin-tarball" = builtins_fetchTarball;
       "file" = pkgs.fetchurl;
+      "builtin-url" = builtins_fetchurl;
     };
 };
 # NOTE: spec must _not_ have an "outPath" attribute
@@ -62,6 +84,6 @@ mapAttrs (_: spec:
     if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
     then
       spec //
-      { outPath = getFetcher spec { inherit (spec) url sha256; } ; }
+      { outPath = callFunctionWith spec (getFetcher spec) { }; }
     else spec
   ) sources
diff --git a/site/niv.svg b/site/niv.svg
index 79fb047..2162944 100644
--- a/site/niv.svg
+++ b/site/niv.svg
@@ -1,10 +1,10 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="6ec29c22551b78782424968916ba8a67" baseProfile="full" viewBox="0 0 703 523" width="703" version="1.1">
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="1f2ba40b461bae96af1073b4f3fbfdf2" baseProfile="full" viewBox="0 0 703 523" width="703" version="1.1">
     <defs>
         <termtosvg:template_settings xmlns:termtosvg="https://github.com/nbedos/termtosvg">
             <termtosvg:screen_geometry columns="82" rows="26"/>
         </termtosvg:template_settings>
         <style type="text/css" id="generated-style"><![CDATA[:root {
-            --animation-duration: 7000ms;
+            --animation-duration: 5500ms;
         }
 
     #screen {
@@ -44,5 +44,5 @@
     <circle cx="44" cy="23" r="7" class="color3"/>
     <circle cx="64" cy="23" r="7" class="color2"/>
     <svg id="screen" width="656" x="23" y="50" viewBox="0 0 656 442" preserveAspectRatio="xMidYMin meet">
-        <rect class="background" height="100%" width="100%" x="0" y="0"/><g display="none"><rect class="foreground" height="17" width="8" x="0" y="85"/><use y="85" xlink:href="#g1"/><animate attributeName="display" begin="0ms; anim_last.end" dur="1500ms" from="inline" to="inline"/></g><g display="none"><rect class="foreground" height="17" width="8" x="0" y="187"/><use y="187" xlink:href="#g1"/><animate attributeName="display" begin="1500ms; anim_last.end+1500ms" dur="1500ms" from="inline" to="inline"/></g><g display="none"><rect class="foreground" height="17" width="8" x="0" y="204"/><use y="204" xlink:href="#g1"/><animate attributeName="display" begin="3000ms; anim_last.end+3000ms" dur="1500ms" from="inline" to="inline"/></g><g display="none"><rect class="foreground" height="17" width="8" x="0" y="306"/><use y="306" xlink:href="#g1"/><animate attributeName="display" begin="4500ms; anim_last.end+4500ms" dur="1500ms" from="inline" to="inline"/></g><g display="none"><use y="0" xlink:href="#g2"/><use y="17" xlink:href="#g3"/><use y="34" xlink:href="#g4"/><use y="51" xlink:href="#g5"/><use y="68" xlink:href="#g6"/><animate attributeName="display" begin="0ms; anim_last.end" dur="7000ms" from="inline" to="inline"/></g><g display="none"><use y="85" xlink:href="#g7"/><use y="102" xlink:href="#g8"/><use y="119" xlink:href="#g9"/><use y="136" xlink:href="#g10"/><use y="153" xlink:href="#g11"/><use y="170" xlink:href="#g6"/><animate attributeName="display" begin="1500ms; anim_last.end+1500ms" dur="5500ms" from="inline" to="inline"/></g><g display="none"><use y="187" xlink:href="#g7"/><animate attributeName="display" begin="3000ms; anim_last.end+3000ms" dur="4000ms" from="inline" to="inline"/></g><g display="none"><use y="272" xlink:href="#g12"/><use y="289" xlink:href="#g6"/><use y="204" xlink:href="#g13"/><use y="221" xlink:href="#g14"/><use y="238" xlink:href="#g10"/><animate attributeName="display" begin="4500ms; anim_last.end+4500ms" dur="2500ms" from="inline" to="inline"/></g><g display="none"><use y="306" xlink:href="#g7"/><use y="323" xlink:href="#g15"/><use y="340" xlink:href="#g16"/><use y="357" xlink:href="#g10"/><rect class="foreground" height="17" width="8" x="0" y="374"/><use y="374" xlink:href="#g1"/><animate attributeName="display" begin="6000ms; anim_last.end+6000ms" dur="1000ms" from="inline" to="inline" id="anim_last"/></g><defs><g id="g1"><text class="background" textLength="8" x="0"> </text></g><g id="g2"><text class="foreground" textLength="80" x="0">$ niv init</text></g><g id="g3"><text class="foreground" textLength="192" x="0">Creating nix/sources.nix</text></g><g id="g4"><text class="foreground" textLength="200" x="0">Creating nix/sources.json</text></g><g id="g5"><text class="foreground" textLength="152" x="0">Importing 'niv' ...</text></g><g id="g6"><text class="foreground" textLength="160" x="0">Reading sources file</text></g><g id="g7"><text class="foreground" textLength="96" x="0">unpacking...</text></g><g id="g8"><text class="foreground" textLength="656" x="0">path is '/nix/store/z2dh6cm4jk6x4bwvrbl9lr4drglnms78-5d9e3a5f7d51765f0369a4682770e</text></g><g id="g9"><text class="foreground" textLength="152" x="0">c57d863f19f.tar.gz'</text></g><g id="g10"><text class="foreground" textLength="192" x="0">Writing new sources file</text></g><g id="g11"><text class="foreground" textLength="184" x="0">Importing 'nixpkgs' ...</text></g><g id="g12"><text class="foreground" textLength="168" x="0">$ niv add stedolan/jq</text></g><g id="g13"><text class="foreground" textLength="656" x="0">path is '/nix/store/qbzbhgq78m94j4dm026y7mi7nkd4lgh4-a7e559a5504572008567383c3dc8e</text></g><g id="g14"><text class="foreground" textLength="152" x="0">142fa7a8633.tar.gz'</text></g><g id="g15"><text class="foreground" textLength="656" x="0">path is '/nix/store/yjz2v8kfk2jkzc0w7lh43hfmcafpqs33-ad9fc9f559e78a764aac20f669f23</text></g><g id="g16"><text class="foreground" textLength="152" x="0">cdd020cd943.tar.gz'</text></g></defs></svg>
+        <rect class="background" height="100%" width="100%" x="0" y="0"/><g display="none"><rect class="foreground" height="17" width="8" x="0" y="85"/><use y="85" xlink:href="#g1"/><animate attributeName="display" begin="0ms; anim_last.end" dur="1500ms" from="inline" to="inline"/></g><g display="none"><rect class="foreground" height="17" width="8" x="0" y="187"/><use y="187" xlink:href="#g1"/><animate attributeName="display" begin="1500ms; anim_last.end+1500ms" dur="1500ms" from="inline" to="inline"/></g><g display="none"><rect class="foreground" height="17" width="8" x="0" y="306"/><use y="306" xlink:href="#g1"/><animate attributeName="display" begin="3000ms; anim_last.end+3000ms" dur="1500ms" from="inline" to="inline"/></g><g display="none"><use y="0" xlink:href="#g2"/><use y="17" xlink:href="#g3"/><use y="34" xlink:href="#g4"/><use y="51" xlink:href="#g5"/><use y="68" xlink:href="#g6"/><animate attributeName="display" begin="0ms; anim_last.end" dur="5500ms" from="inline" to="inline"/></g><g display="none"><use y="85" xlink:href="#g7"/><use y="102" xlink:href="#g8"/><use y="119" xlink:href="#g9"/><use y="136" xlink:href="#g10"/><use y="153" xlink:href="#g11"/><use y="170" xlink:href="#g6"/><animate attributeName="display" begin="1500ms; anim_last.end+1500ms" dur="4000ms" from="inline" to="inline"/></g><g display="none"><use y="272" xlink:href="#g12"/><use y="289" xlink:href="#g6"/><use y="187" xlink:href="#g7"/><use y="204" xlink:href="#g13"/><use y="221" xlink:href="#g14"/><use y="238" xlink:href="#g10"/><animate attributeName="display" begin="3000ms; anim_last.end+3000ms" dur="2500ms" from="inline" to="inline"/></g><g display="none"><use y="306" xlink:href="#g7"/><use y="323" xlink:href="#g15"/><use y="340" xlink:href="#g16"/><use y="357" xlink:href="#g10"/><rect class="foreground" height="17" width="8" x="0" y="374"/><use y="374" xlink:href="#g1"/><animate attributeName="display" begin="4500ms; anim_last.end+4500ms" dur="1000ms" from="inline" to="inline" id="anim_last"/></g><defs><g id="g1"><text class="background" textLength="8" x="0"> </text></g><g id="g2"><text class="foreground" textLength="80" x="0">$ niv init</text></g><g id="g3"><text class="foreground" textLength="192" x="0">Creating nix/sources.nix</text></g><g id="g4"><text class="foreground" textLength="200" x="0">Creating nix/sources.json</text></g><g id="g5"><text class="foreground" textLength="152" x="0">Importing 'niv' ...</text></g><g id="g6"><text class="foreground" textLength="160" x="0">Reading sources file</text></g><g id="g7"><text class="foreground" textLength="96" x="0">unpacking...</text></g><g id="g8"><text class="foreground" textLength="656" x="0">path is '/nix/store/d2ibkfdbsqkr6avllvkqcwdyj771qmvv-30f55f14e1580325f65dd2bd0be4e</text></g><g id="g9"><text class="foreground" textLength="152" x="0">bd0797a8c83.tar.gz'</text></g><g id="g10"><text class="foreground" textLength="192" x="0">Writing new sources file</text></g><g id="g11"><text class="foreground" textLength="184" x="0">Importing 'nixpkgs' ...</text></g><g id="g12"><text class="foreground" textLength="168" x="0">$ niv add stedolan/jq</text></g><g id="g13"><text class="foreground" textLength="656" x="0">path is '/nix/store/qbzbhgq78m94j4dm026y7mi7nkd4lgh4-a7e559a5504572008567383c3dc8e</text></g><g id="g14"><text class="foreground" textLength="152" x="0">142fa7a8633.tar.gz'</text></g><g id="g15"><text class="foreground" textLength="656" x="0">path is '/nix/store/yjz2v8kfk2jkzc0w7lh43hfmcafpqs33-ad9fc9f559e78a764aac20f669f23</text></g><g id="g16"><text class="foreground" textLength="152" x="0">cdd020cd943.tar.gz'</text></g></defs></svg>
 </svg>
\ No newline at end of file