diff --git a/lib/deferred_config.ex b/lib/deferred_config.ex
index b0ab201..247c929 100644
--- a/lib/deferred_config.ex
+++ b/lib/deferred_config.ex
@@ -1,4 +1,4 @@
-defmodule DeferredConfig do  
+defmodule DeferredConfig do
   @moduledoc """
   Seamlessly add runtime config to your library, with the
   "system tuples" or the `{m,f,a}` patterns.
@@ -15,27 +15,27 @@ defmodule DeferredConfig do
       end
 
   Where `:mine` is the name of your OTP app.
-  
+
   Now you and users of your app or lib can configure
   as follows, and it'll work -- regardless of if they're
   running it from iex, or a release with env vars set:
-  
-      config :mine, 
-      
+
+      config :mine,
+
         # string from env var, or `nil` if missing.
         port1: {:system, "PORT"},
 
         # string from env var |> integer; `nil` if missing.
         port2: {:system, "PORT", {String, :to_integer}},
-        
+
         # string from env var, or "4000" as default.
         port3: {:system, "PORT", "4000"},
-        
+
         # converts env var to integer, or 4000 as default.
         port4: {:system, "PORT", 4000, {String, :to_integer}}
-  
+
   **Accessing config does not change.**
-  
+
   Since you can use arbitrary transformation functions,
   you can do advanced transformations if you need to:
 
@@ -50,11 +50,11 @@ defmodule DeferredConfig do
         end
       end
 
-      # config.exs 
+      # config.exs
       config :my_app,
         port: {:system, "MY_IP", {127,0,0,1}, {Mine.Ip, :str2ip}
 
-  See `README.md` for explanation of rationale. 
+  See `README.md` for explanation of rationale.
   **TL;DR:** `REPLACE_OS_VARS` is string-only and release-only,
   and `{:system, ...}` support among libraries is spotty
   and easy to get wrong in ways that bite your users
@@ -65,19 +65,17 @@ defmodule DeferredConfig do
   """
   require Logger
   import ReplacingWalk, only: [walk: 3]
-  
+
   @default_rts [
-    {&DeferredConfig.recognize_system_tuple/1,
-     &DeferredConfig.get_system_tuple/1},
-    {&DeferredConfig.recognize_mfa_tuple/1,
-     &DeferredConfig.transform_mfa_tuple/1}
+    {&DeferredConfig.recognize_system_tuple/1, &DeferredConfig.get_system_tuple/1},
+    {&DeferredConfig.recognize_mfa_tuple/1, &DeferredConfig.transform_mfa_tuple/1}
   ]
-  
+
   @doc """
   Populate deferred values in an app's config.
   Best run during `Application.start/2`.
 
-  **By default** attempts to populate the common 
+  **By default** attempts to populate the common
   `{:system, "VAR"}` tuple form for getting values from
   `System.get_env/1`, and the more
   general `{:apply, {Mod, fun, [args]}}` form as well.
@@ -86,25 +84,26 @@ defmodule DeferredConfig do
   defaults and conversion functions, see
   `Peerage.DeferredConfig.get_system_tuple/1`.
 
-  Can be extended by passing in a different 
-  enumerable of `{&recognizer/1, &transformer/1}` 
+  Can be extended by passing in a different
+  enumerable of `{&recognizer/1, &transformer/1}`
   functions.
   """
   def populate(app, transforms \\ @default_rts) do
-    :ok = app
-    |> Application.get_all_env
-    |> transform_cfg(transforms)
-    |> apply_transformed_cfg!(app)
+    :ok =
+      app
+      |> Application.get_all_env()
+      |> transform_cfg(transforms)
+      |> apply_transformed_cfg!(app)
   end
 
   @doc """
-  Given a config kvlist, and an enumerable of 
+  Given a config kvlist, and an enumerable of
   `{&recognize/1, &transform/1}` functions,
   returns a kvlist with the values transformed
   via replacing walk.
   """
   def transform_cfg(cfg, rts \\ @default_rts) when is_list(rts) do
-    Enum.map(cfg, fn {k,v} ->
+    Enum.map(cfg, fn {k, v} ->
       {k, apply_rts(v, rts)}
     end)
   end
@@ -112,7 +111,7 @@ defmodule DeferredConfig do
   @doc "`Application.put_env/3` for config kvlist"
   def apply_transformed_cfg!(kvlist, app) do
     kvlist
-    |> Enum.each(fn {k,v} ->
+    |> Enum.each(fn {k, v} ->
       Application.put_env(app, k, v)
     end)
   end
@@ -126,6 +125,7 @@ defmodule DeferredConfig do
 
   # apply sequence of replacing walks to a value
   defp apply_rts(val, []), do: val
+
   defp apply_rts(val, rts) when is_list(rts) do
     Enum.reduce(rts, val, fn {r, t}, acc_v ->
       walk(acc_v, r, t)
@@ -136,20 +136,24 @@ defmodule DeferredConfig do
   Recognize mfa tuple, like `{:apply, {File, :read!, ["name"]}}`.
   Returns `true` on recognition, `false` otherwise.
   """
-  def recognize_mfa_tuple({:apply, {m,f,a}})
-  when is_atom(m) and is_atom(f) and is_list(a),
-    do: true
+  def recognize_mfa_tuple({:apply, {m, f, a}})
+      when is_atom(m) and is_atom(f) and is_list(a),
+      do: true
+
   def recognize_mfa_tuple({:apply, t}) do
-    Logger.error "badcfg - :apply needs {:m, :f, lst}. "<>
-      "given: #{ inspect t }"
+    Logger.error(
+      "badcfg - :apply needs {:m, :f, lst}. " <>
+        "given: #{inspect(t)}"
+    )
+
     false
   end
+
   def recognize_mfa_tuple(_), do: false
-  
+
   @doc "Return evaluated `{:apply, {mod, fun, args}}` tuple."
-  def transform_mfa_tuple({:apply, {m,f,a}}), do: apply(m,f,a)
+  def transform_mfa_tuple({:apply, {m, f, a}}), do: apply(m, f, a)
 
-  
   @doc """
   Recognizer for system tuples of forms:
   - `{:system, "VAR"}`
@@ -158,28 +162,31 @@ defmodule DeferredConfig do
   - `{:system, "VAR", default_value, {String, :to_integer}}`
   Returns `true` when it matches one, `false` otherwise.
   """
-  def recognize_system_tuple({:system, ""<>_k}),           do: true
-  def recognize_system_tuple({:system, ""<>_k, _default}), do: true
-  def recognize_system_tuple({:system, ""<>_k, _d, _mf}),  do: true
-  def recognize_system_tuple(_),                               do: false
+  def recognize_system_tuple({:system, "" <> _k}), do: true
+  def recognize_system_tuple({:system, "" <> _k, _default}), do: true
+  def recognize_system_tuple({:system, "" <> _k, _d, _mf}), do: true
+  def recognize_system_tuple(_), do: false
 
   @doc """
   Return transformed copy of recognized system tuples:
-  gets from env, optionally converts it, with 
+  gets from env, optionally converts it, with
   optional default if env returned nothing.
   """
   def get_system_tuple({:system, k}), do: System.get_env(k)
+
   def get_system_tuple({:system, k, {m, f}}) do
-    apply m, f, [ System.get_env(k) ]
+    apply(m, f, [System.get_env(k)])
   end
+
   def get_system_tuple({:system, k, d}), do: System.get_env(k) || d
+
   def get_system_tuple({:system, k, d, {m, f}}) do
-    (val = System.get_env k) && apply(m, f, [val]) || d
+    # (val = System.get_env k) && apply(m, f, [val]) || d
+    case System.get_env(k) do
+      nil -> d
+      val -> apply(m, f, [val])
+    end
   end
-  def get_system_tuple(t), do: throw "Could not fetch: #{inspect t}"
 
-  
-  
-    
+  def get_system_tuple(t), do: throw("Could not fetch: #{inspect(t)}")
 end
-
diff --git a/mix.lock b/mix.lock
index 0e99ff4..3a0182d 100644
--- a/mix.lock
+++ b/mix.lock
@@ -1,4 +1,6 @@
-%{"bunt": {:hex, :bunt, "0.1.6", "5d95a6882f73f3b9969fdfd1953798046664e6f77ec4e486e6fafc7caad97c6f", [:mix], []},
-  "credo": {:hex, :credo, "0.5.3", "0c405b36e7651245a8ed63c09e2d52c2e2b89b6d02b1570c4d611e0fcbecf4a2", [:mix], [{:bunt, "~> 0.1.6", [hex: :bunt, optional: false]}]},
-  "earmark": {:hex, :earmark, "1.0.3", "89bdbaf2aca8bbb5c97d8b3b55c5dd0cff517ecc78d417e87f1d0982e514557b", [:mix], []},
-  "ex_doc": {:hex, :ex_doc, "0.14.5", "c0433c8117e948404d93ca69411dd575ec6be39b47802e81ca8d91017a0cf83c", [:mix], [{:earmark, "~> 1.0", [hex: :earmark, optional: false]}]}}
+%{
+  "bunt": {:hex, :bunt, "0.1.6", "5d95a6882f73f3b9969fdfd1953798046664e6f77ec4e486e6fafc7caad97c6f", [:mix], [], "hexpm", "4fb7b2f7b04af13cf210b132f8d10db52d4a57d36cb974e8025d7fdb12ca97fc"},
+  "credo": {:hex, :credo, "0.5.3", "0c405b36e7651245a8ed63c09e2d52c2e2b89b6d02b1570c4d611e0fcbecf4a2", [:mix], [{:bunt, "~> 0.1.6", [hex: :bunt, repo: "hexpm", optional: false]}], "hexpm", "90bca5180fe64c47343969ad3e32bf1edc18d929689e8c00c810655a7a391427"},
+  "earmark": {:hex, :earmark, "1.0.3", "89bdbaf2aca8bbb5c97d8b3b55c5dd0cff517ecc78d417e87f1d0982e514557b", [:mix], [], "hexpm", "0fdcd651f9689e81cda24c8e5d06947c5aca69dbd8ce3d836b02bcd0c6004592"},
+  "ex_doc": {:hex, :ex_doc, "0.14.5", "c0433c8117e948404d93ca69411dd575ec6be39b47802e81ca8d91017a0cf83c", [:mix], [{:earmark, "~> 1.0", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm", "5c30e436a5acfdc2fd8fe6866585fcaf30f434c611d8119d4f3390ced2a550f3"},
+}
diff --git a/test/deferred_config_test.exs b/test/deferred_config_test.exs
index a7eb89f..bd14b52 100644
--- a/test/deferred_config_test.exs
+++ b/test/deferred_config_test.exs
@@ -3,29 +3,30 @@ defmodule DeferredConfigTest do
   doctest DeferredConfig
 
   @app :lazy_cfg_test_appname
-  
+
   defmodule MyMod do
-    def get_my_key(""<>bin), do: "your key is 1234. write it down."
+    def get_my_key("" <> bin), do: "your key is 1234. write it down."
   end
-  
+
   setup do
     delete_all_env(@app)
     # give each test a fake env that looks like this
     env = %{"PORT" => "4000"}
+
     system_transform = fn
-      {:system, k}            -> Map.get(env, k)
-      {:system, k, {m, f}}    -> apply m, f, [Map.get(env, k)]
-      {:system, k, d}         -> Map.get(env, k, d)
-      {:system, k, d, {m, f}} -> apply( m, f, [Map.get(env, k)]) || d
+      {:system, k} -> Map.get(env, k)
+      {:system, k, {m, f}} -> apply(m, f, [Map.get(env, k)])
+      {:system, k, d} -> Map.get(env, k, d)
+      {:system, k, d, {m, f}} -> apply(m, f, [Map.get(env, k)]) || d
     end
+
     # our mock stack -- only changes env var retrieval
     transforms = [
       {&DeferredConfig.recognize_system_tuple/1, system_transform},
-      {&DeferredConfig.recognize_mfa_tuple/1,
-       &DeferredConfig.transform_mfa_tuple/1}
+      {&DeferredConfig.recognize_mfa_tuple/1, &DeferredConfig.transform_mfa_tuple/1}
     ]
-    [transforms: transforms,
-     system_transform: system_transform]
+
+    [transforms: transforms, system_transform: system_transform]
   end
 
   test "system tuples support", %{system_transform: transform} do
@@ -34,12 +35,15 @@ defmodule DeferredConfigTest do
       port2: {:system, "PORT", "1111"},
       port3: {:system, "FAIL", "1111"},
       port4: {:system, "PORT", {String, :to_integer}},
-      port5: [{:system, "PORT", 3000, {String, :to_integer}}],
+      port5: [{:system, "PORT", 3000, {String, :to_integer}}]
     ]
-    actual = cfg
-    |> DeferredConfig.transform_cfg([
-      {&DeferredConfig.recognize_system_tuple/1, transform}
-    ])
+
+    actual =
+      cfg
+      |> DeferredConfig.transform_cfg([
+        {&DeferredConfig.recognize_system_tuple/1, transform}
+      ])
+
     assert actual[:port1] == "4000"
     assert actual[:port2] == "4000"
     assert actual[:port3] == "1111"
@@ -47,7 +51,7 @@ defmodule DeferredConfigTest do
     assert actual[:port5] == [4000]
     actual |> DeferredConfig.apply_transformed_cfg!(@app)
 
-    actual = Application.get_all_env @app
+    actual = Application.get_all_env(@app)
     assert actual[:port1] == "4000"
     assert actual[:port2] == "4000"
     assert actual[:port3] == "1111"
@@ -56,31 +60,46 @@ defmodule DeferredConfigTest do
   end
 
   test "non-existent tuple values are handled" do
-    r = DeferredConfig.transform_cfg([key: {:system, "ASDF"}])
+    r = DeferredConfig.transform_cfg(key: {:system, "ASDF"})
     assert r[:key] == nil
   end
+
+  test "boolean environment variable `false` overrides default" do
+    System.put_env("BOOLEAN", "false")
+
+    cfg = [
+      bool_conf: {:system, "BOOLEAN", true, {String, :to_atom}}
+    ]
+
+    actual = DeferredConfig.transform_cfg(cfg)
+
+    assert actual[:bool_conf] === false
+  end
+
   test "readme sys/mfa example", %{transforms: transforms} do
     readme_example = [
-      http: %{ # even inside nested data
+      # even inside nested data
+      http: %{
         # the common 'system tuple' pattern is fully supported
         port: {:system, "PORT", {String, :to_integer}}
       },
       # more general 'mfa tuple' pattern is also supported
       key: {:apply, {MyMod, :get_my_key, ["arg"]}}
     ]
-    actual = readme_example
-    |> DeferredConfig.transform_cfg(transforms)
-    
-    assert "your key is"<>_ = actual[:key]
+
+    actual =
+      readme_example
+      |> DeferredConfig.transform_cfg(transforms)
+
+    assert "your key is" <> _ = actual[:key]
     assert actual[:http][:port] == 4000
   end
-      
+
   defp delete_all_env(app) do
     app
-    |> Application.get_all_env
+    |> Application.get_all_env()
     |> Enum.each(fn {k, v} ->
-      Application.delete_env( app, k )
+      Application.delete_env(app, k)
     end)
   end
-  
 end