Custom changeset validations return error protocol Enumerable not implemented for #Ecto.Changeset

I’m having an issue with my custom changeset validations. There is a form where a user can change their White Labeling colors for their app. I have created custom validations that make sure the colors submitted are either hexdecimal or rgb format. Validation works OK if the value being passed to the validation doesn’t contain # or rgb in the string. If it does, the validations crash the app.

I get this error:

protocol Enumerable not implemented for '#Ecto.Changeset<action: nil, changes: %{background: "#ff", header_background: "#ff"}, errors: [background: {"Color must be in hexdecimal or rgb format", [validation: :format]}], data: #App.WhiteLabels.Theme<>, valid?: false>' of type Ecto.Changeset (a struct). This protocol is implemented for the following type(s): Ecto.Adapters.SQL.Stream, Postgrex.Stream, Floki.HTMLTree, DBConnection.PrepareStream, DBConnection.Stream, StreamData, File.Stream, Map, List, IO.Stream, Range, HashDict, Stream, HashSet, Function, Date.Range, MapSet, GenEvent.Stream

Initial changeset being passed in looks like this

#Ecto.Changeset<
  action: nil,
  changes: %{background: "#ff", header_background: "#ff"},
  errors: [],
  data: #App.WhiteLabels.Theme<>,
  valid?: true
>

Here is a simplified version of my changeset code:

  @spec changeset(%__MODULE__{}, map) :: %Ecto.Changeset{}
  def changeset(schema, attrs) do
    schema
    |> cast(attrs, @fields)
    |> validate_required(@required_fields)
    |> validate_theme_change(:background)
    |> validate_theme_change(:header_background)
  end

  def validate_theme_change(changeset, field) do
    validate_change(changeset, field, fn (current_field, value) ->
      cond do
        String.contains?(value, "http") -> []
        String.contains?(value, "#") -> validate_hexdecimal(changeset, field) |> IO.inspect()
        String.contains?(value, "rgb") -> validate_rgb(changeset, field)
        true -> [{field, "Color must be in hexdecimal or rgb format"}]
      end
    end
    )
  end

  defp validate_hexdecimal(changeset, field) do
    validate_format(changeset, field, ~r/^([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/, message: "Color must be in hexdecimal or rgb format")
  end

  defp validate_rgb(changeset, field) do
    validate_format(changeset, field, ~r/^[Rr][Gg][Bb][\(](((([\d]{1,3})[\,]{0,1})[\s]*){3})[\)]$/, message: "Color must be in hexdecimal or rgb format")
  end

Stack trace shows it gets to the line |> validate_theme_change(:background)