diff --git a/lib/isn/base.ex b/lib/isn/base.ex
index 3c2222e..6bdb63e 100644
--- a/lib/isn/base.ex
+++ b/lib/isn/base.ex
@@ -13,8 +13,11 @@ defmodule Isn.Base do
 
       defdelegate blank?, to: Ecto.Type
 
+      def cast(nil), do: :error
       def cast(isn), do: {:ok, to_string(isn)}
+
       def load(isn), do: {:ok, to_string(isn)}
+
       def dump(isn), do: {:ok, to_string(isn)}
     end
   end
diff --git a/test/isn/isbn13_test.exs b/test/isn/isbn13_test.exs
index 753965a..8d1daf8 100644
--- a/test/isn/isbn13_test.exs
+++ b/test/isn/isbn13_test.exs
@@ -1,7 +1,18 @@
 defmodule Isn.ISBN13Test do
-  use ExUnit.Case
+  use ExUnit.Case, async: true
 
-  # dump
-  # cast
-  # load
+  @test_isbn "978-1-937785-58-1"
+
+  test "cast" do
+    assert Isn.ISBN13.cast(@test_isbn) == {:ok, @test_isbn}
+    assert Isn.ISBN13.cast(nil) == :error
+  end
+
+  test "load" do
+    assert Isn.ISBN13.load(@test_isbn) == {:ok, @test_isbn}
+  end
+
+  test "dump" do
+    assert Isn.ISBN13.dump(@test_isbn) == {:ok, @test_isbn}
+  end
 end