diff --git a/README.md b/README.md
index f1051af..889333d 100644
--- a/README.md
+++ b/README.md
@@ -98,7 +98,7 @@ To `use` `EctoEnum` with string-backed storage:
 
 ```elixir
 defmodule CustomEnum do
-  use EctoEnum, "ready", "set", "go"
+  use EctoEnum, ["ready", "set", "go"]
 end
 ```
 
diff --git a/lib/ecto_enum.ex b/lib/ecto_enum.ex
index 1f2606d..d7359d6 100644
--- a/lib/ecto_enum.ex
+++ b/lib/ecto_enum.ex
@@ -127,19 +127,6 @@ defmodule EctoEnum do
   defmacro defenum(module, enum) do
     quote do
       enum = Macro.escape(unquote(enum))
-      [h | _t] = enum
-
-      enum =
-        cond do
-          Keyword.keyword?(enum) ->
-            enum
-
-          is_binary(h) ->
-            Enum.map(enum, fn value -> {String.to_atom(value), value} end)
-
-          true ->
-            raise "Enum must be a keyword list or a list of strings"
-        end
 
       defmodule unquote(module) do
         use EctoEnum.Use, enum
diff --git a/lib/ecto_enum/use.ex b/lib/ecto_enum/use.ex
index 9d968d0..bdedd54 100644
--- a/lib/ecto_enum/use.ex
+++ b/lib/ecto_enum/use.ex
@@ -5,6 +5,20 @@ defmodule EctoEnum.Use do
 
   defmacro __using__(opts) do
     quote bind_quoted: [opts: opts] do
+      [h | _t] = opts
+
+      opts =
+        cond do
+          Keyword.keyword?(opts) ->
+            opts
+
+          is_binary(h) ->
+            Enum.map(opts, fn value -> {String.to_atom(value), value} end)
+
+          true ->
+            raise "Enum must be a keyword list or a list of strings"
+        end
+
       typespec = Typespec.make(Keyword.keys(opts))
 
       @behaviour Ecto.Type
@@ -35,6 +49,12 @@ defmodule EctoEnum.Use do
 
       def cast(_other), do: :error
 
+      for {key, value} <- opts, k <- Enum.uniq([key, value, Atom.to_string(key)]) do
+        def cast!(unquote(k)), do: unquote(key)
+      end
+
+      def cast!(other), do: raise Ecto.CastError, type: __MODULE__, value: other
+
       for {key, value} <- opts, k <- Enum.uniq([key, value, Atom.to_string(key)]) do
         def dump(unquote(k)), do: {:ok, unquote(value)}
       end
@@ -47,6 +67,18 @@ defmodule EctoEnum.Use do
         raise Ecto.ChangeError, message: msg
       end
 
+      for {key, value} <- opts, k <- Enum.uniq([key, value, Atom.to_string(key)]) do
+        def dump!(unquote(k)), do: unquote(value)
+      end
+
+      def dump!(term) do
+        msg =
+          "Value `#{inspect(term)}` is not a valid enum for `#{inspect(__MODULE__)}`. " <>
+            "Valid enums are `#{inspect(__valid_values__())}`"
+
+        raise Ecto.ChangeError, message: msg
+      end
+
       def embed_as(_), do: :self
 
       def equal?(term1, term2), do: term1 == term2