PostGIS UndefinedFunctionError (Geo.PostGIS.Geometry.equal?/2)

Attempting to insert %Geo.Multipoint{} struct into the db. When casting my changeset, I get the following error message (UndefinedFunctionError) function Geo.PostGIS.Geometry.equal?/2 is undefined or private.

My code is as follows:

defmodule HeatMap do
    use Ecto.Schema
    import Ecto.Changeset

    schema "heat_maps" do
        field :name, :string
        field :geometry, Geo.PostGIS.Geometry

        timestamps()
    end

    @doc false
    def changeset(map, attrs) do
        map
        |> cast(attrs, [:name, :geometry])
        |> validate_required([:name, :geometry])
    end
end

# Error happens here
HeatMap.changeset(
    %HeatMap{},
    %{
        name: "Some Map",
        geometry: %Geo.MultiPoint{
              coordinates: [{-73, 40}],
              srid: 4326
        }
    }
)

Has anyone encountered this problem? How can I go about making Ecto.Changeset happy?