diff --git a/lib/event_bus/models/event.ex b/lib/event_bus/models/event.ex
index 8657048..f2ca16b 100644
--- a/lib/event_bus/models/event.ex
+++ b/lib/event_bus/models/event.ex
@@ -52,7 +52,7 @@ defmodule EventBus.Model.Event do
     occurred_at - initialized_at
   end
 
-  def duration(_) do
+  def duration(%__MODULE__{}) do
     0
   end
 end
diff --git a/mix.exs b/mix.exs
index 1b4a92b..5883adb 100644
--- a/mix.exs
+++ b/mix.exs
@@ -43,7 +43,7 @@ defmodule EventBus.Mixfile do
       {:credo, "~> 0.10.0", only: [:dev]},
       {:dialyxir, "~> 1.0.0-rc.3", only: [:dev], runtime: false},
       {:excoveralls, "~> 0.9.1", only: [:test]},
-      {:ex_doc, "~> 0.18.1", only: [:dev]}
+      {:ex_doc, "~> 0.18.4", only: [:dev]}
     ]
   end
 
diff --git a/mix.lock b/mix.lock
index 3d550fc..d73e53d 100644
--- a/mix.lock
+++ b/mix.lock
@@ -3,8 +3,8 @@
   "certifi": {:hex, :certifi, "2.3.1", "d0f424232390bf47d82da8478022301c561cf6445b5b5fb6a84d49a9e76d2639", [:rebar3], [{:parse_trans, "3.2.0", [hex: :parse_trans, repo: "hexpm", optional: false]}], "hexpm"},
   "credo": {:hex, :credo, "0.10.0", "66234a95effaf9067edb19fc5d0cd5c6b461ad841baac42467afed96c78e5e9e", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm"},
   "dialyxir": {:hex, :dialyxir, "1.0.0-rc.3", "774306f84973fc3f1e2e8743eeaa5f5d29b117f3916e5de74c075c02f1b8ef55", [:mix], [], "hexpm"},
-  "earmark": {:hex, :earmark, "1.2.4", "99b637c62a4d65a20a9fb674b8cffb8baa771c04605a80c911c4418c69b75439", [:mix], [], "hexpm"},
-  "ex_doc": {:hex, :ex_doc, "0.18.3", "f4b0e4a2ec6f333dccf761838a4b253d75e11f714b85ae271c9ae361367897b7", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"},
+  "earmark": {:hex, :earmark, "1.2.5", "4d21980d5d2862a2e13ec3c49ad9ad783ffc7ca5769cf6ff891a4553fbaae761", [:mix], [], "hexpm"},
+  "ex_doc": {:hex, :ex_doc, "0.18.4", "4406b8891cecf1352f49975c6d554e62e4341ceb41b9338949077b0d4a97b949", [:mix], [{:earmark, "~> 1.1", [hex: :earmark, repo: "hexpm", optional: false]}], "hexpm"},
   "excoveralls": {:hex, :excoveralls, "0.9.1", "14fd20fac51ab98d8e79615814cc9811888d2d7b28e85aa90ff2e30dcf3191d6", [:mix], [{:hackney, ">= 0.12.0", [hex: :hackney, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm"},
   "exjsx": {:hex, :exjsx, "4.0.0", "60548841e0212df401e38e63c0078ec57b33e7ea49b032c796ccad8cde794b5c", [:mix], [{:jsx, "~> 2.8.0", [hex: :jsx, repo: "hexpm", optional: false]}], "hexpm"},
   "hackney": {:hex, :hackney, "1.13.0", "24edc8cd2b28e1c652593833862435c80661834f6c9344e84b6a2255e7aeef03", [:rebar3], [{:certifi, "2.3.1", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "5.1.2", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "1.0.2", [hex: :mimerl, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.1", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm"},