diff --git a/README.md b/README.md
index 25f16e9..38c8da0 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,7 @@ by adding `xler` to your list of dependencies in `mix.exs`:
 ```elixir
 def deps do
   [
-    {:xler, "~> 0.1.0"}
+    {:xler, "~> 0.2.0"}
   ]
 end
 ```
diff --git a/lib/xler.ex b/lib/xler.ex
index 8327c89..3d358ff 100644
--- a/lib/xler.ex
+++ b/lib/xler.ex
@@ -14,7 +14,8 @@ defmodule Xler do
       {:ok, ["Sheet 1"]}
 
   """
-  def worksheets(filename), do: filename |> Native.worksheets()
+  def worksheets(filename) when is_binary(filename), do: filename |> Native.worksheets()
+  def worksheets(_), do: {:error, "not a string"}
 
   @doc """
   Parses a specific worksheet from a file
@@ -27,5 +28,8 @@ defmodule Xler do
       {:ok, [["Date", "Text"]]}
 
   """
-  def parse(filename, worksheet), do: filename |> Native.parse(worksheet)
+  def parse(filename, worksheet) when is_binary(filename) and is_binary(worksheet),
+    do: filename |> Native.parse(worksheet)
+
+  def parse(_, _), do: {:error, "not a string"}
 end
diff --git a/lib/xler/native.ex b/lib/xler/native.ex
index f478144..034146f 100644
--- a/lib/xler/native.ex
+++ b/lib/xler/native.ex
@@ -1,5 +1,6 @@
 defmodule Xler.Native do
   use Rustler, otp_app: :xler, crate: :xler_native
+  @moduledoc false
 
   def parse(_filename, _worksheet), do: error()
   def worksheets(_filename), do: error()
diff --git a/mix.exs b/mix.exs
index 0db4249..44dedb4 100644
--- a/mix.exs
+++ b/mix.exs
@@ -4,7 +4,7 @@ defmodule Xler.MixProject do
   def project do
     [
       app: :xler,
-      version: "0.1.1",
+      version: "0.2.3",
       elixir: "~> 1.8",
       start_permanent: Mix.env() == :prod,
       compilers: [:rustler] ++ Mix.compilers(),