Argument Error in SAML integration

Please i’m having issue getting Samly(Samly - Add SAML SSO to your Phoenix application (now with multiple identity provider support)) to work in my phoenix app. I have installed as stated in the readme details.

Below is what my config looks like

config :samly, Samly.Provider,
  idp_id_from: :path_segment,
  service_providers: [
    %{
      id: "entsp",
      certfile: "priv/keys/samly.crt",
      keyfile: "priv/keys/samly.pem"
    }
  ],
  identity_providers: [
    %{
      id: "entidp",
      sp_id: "entsp",
      metadata_file: "idp_metadata.xml"
    }
  ]

While testing the login with the link http://localhost:4000/sso/auth/signin/entidp, i got Argument error

[error] #PID<0.704.0> running EnterpriseWeb.Endpoint (cowboy_protocol) terminated
Server: localhost:4000 (http)
Request: POST /sso/auth/signin/entidp
** (exit) an exception was raised:
    ** (ArgumentError) argument error
        :erlang.binary_to_list(nil)
        (xmerl) xmerl_lib.erl:69: :xmerl_lib.export_text/2
        (xmerl) xmerl.erl:191: :xmerl.export_content/2
        (xmerl) xmerl.erl:224: :xmerl.export_element/2
        (xmerl) xmerl.erl:199: :xmerl.export_content/2
        (xmerl) xmerl.erl:199: :xmerl.export_content/2
        (xmerl) xmerl.erl:224: :xmerl.export_element/2
        (xmerl) xmerl.erl:199: :xmerl.export_content/2
        (xmerl) xmerl.erl:168: :xmerl.export1/3
        (esaml) /Library/WebServer/Documents/enterprise/deps/esaml/src/esaml_binding.erl:77: :esaml_binding.encode_http_post/3
        (samly) lib/samly/router_util.ex:94: Samly.RouterUtil.send_saml_request/5
        (samly) lib/samly/auth_router.ex:1: Samly.AuthRouter.plug_builder_call/2
        (plug) lib/plug/router/utils.ex:92: Plug.Router.Utils.forward/4
        (samly) lib/samly/router.ex:1: Samly.Router.plug_builder_call/2
        (phoenix) lib/phoenix/router/route.ex:147: Phoenix.Router.Route.forward/4
        (phoenix) lib/phoenix/router.ex:278: Phoenix.Router.__call__/1
        (enterprise) lib/enterprise_web/endpoint.ex:1: EnterpriseWeb.Endpoint.plug_builder_call/2
        (enterprise) lib/plug/debugger.ex:102: EnterpriseWeb.Endpoint."call (overridable 3)"/2
        (enterprise) lib/enterprise_web/endpoint.ex:1: EnterpriseWeb.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:16: Plug.Adapters.Cowboy.Handler.upgrade/4

Please how can i resolve this? Thanks.