diff --git a/lib/cadet/accounts/authorization.ex b/lib/cadet/accounts/authorization.ex
index 6c8b32f06..fb3493ea1 100644
--- a/lib/cadet/accounts/authorization.ex
+++ b/lib/cadet/accounts/authorization.ex
@@ -23,7 +23,7 @@ defmodule Cadet.Accounts.Authorization do
   @required_fields ~w(provider uid token)a
   @optional_fields ~w(refresh_token expires_at)a
 
-  def changeset(authorization, params \\ :empty) do
+  def changeset(authorization, params \\ %{}) do
     authorization
     |> cast(params, @required_fields ++ @optional_fields)
     |> validate_required(@required_fields)
diff --git a/lib/cadet/accounts/form/registration.ex b/lib/cadet/accounts/form/registration.ex
index 86340d3b0..e697439ea 100644
--- a/lib/cadet/accounts/form/registration.ex
+++ b/lib/cadet/accounts/form/registration.ex
@@ -22,7 +22,7 @@ defmodule Cadet.Accounts.Form.Registration do
 
   @email_format ~r/^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/
 
-  def changeset(registration, params \\ :empty) do
+  def changeset(registration, params \\ %{}) do
     registration
     |> cast(params, @required_fields ++ @optional_fields)
     |> validate_required(@required_fields)
diff --git a/lib/cadet/accounts/user.ex b/lib/cadet/accounts/user.ex
index 6204f79b0..8a2791241 100644
--- a/lib/cadet/accounts/user.ex
+++ b/lib/cadet/accounts/user.ex
@@ -20,7 +20,7 @@ defmodule Cadet.Accounts.User do
   @required_fields ~w(first_name role)a
   @optional_fields ~w(last_name)a
 
-  def changeset(user, params \\ :empty) do
+  def changeset(user, params \\ %{}) do
     user
     |> cast(params, @required_fields ++ @optional_fields)
     |> validate_required(@required_fields)
diff --git a/lib/cadet/course.ex b/lib/cadet/course.ex
index be602ad43..3cbfbfb8a 100644
--- a/lib/cadet/course.ex
+++ b/lib/cadet/course.ex
@@ -189,17 +189,10 @@ defmodule Cadet.Course do
   end
 
   def delete_material(material = %Material{}) do
-    cond do
-      material == nil ->
-        {:error, :not_found}
-
-      material.file != nil ->
-        Upload.delete({material.file, material})
-        Repo.delete(material)
-
-      true ->
-        Repo.delete(material)
+    if material.file do
+      Upload.delete({material.file, material})
     end
+    Repo.delete(material)
   end
 
   @doc """
diff --git a/lib/cadet/course/material.ex b/lib/cadet/course/material.ex
index 4b248742e..efe907522 100644
--- a/lib/cadet/course/material.ex
+++ b/lib/cadet/course/material.ex
@@ -32,8 +32,8 @@ defmodule Cadet.Course.Material do
 
   def changeset(material, attrs \\ %{}) do
     material
-    |> cast(attrs, @required_fields ++ @optional_fields)
     |> cast_attachments(attrs, @optional_file_fields)
+    |> cast(attrs, @required_fields ++ @optional_fields)
     |> validate_changeset
   end
 
diff --git a/lib/cadet/factory.ex b/lib/cadet/factory.ex
index c916454d7..d5458a3bd 100644
--- a/lib/cadet/factory.ex
+++ b/lib/cadet/factory.ex
@@ -3,6 +3,7 @@ defmodule Cadet.Factory do
   Factory for testing
   """
   use ExMachina.Ecto, repo: Cadet.Repo
+  @dialyzer {:no_return, fields_for: 1} # fields_for has been deprecated, only raising exception
 
   alias Cadet.Accounts.User
   alias Cadet.Accounts.Authorization
diff --git a/lib/cadet_web/controllers/session_controller.ex b/lib/cadet_web/controllers/session_controller.ex
index d0ec553e7..cf6bb6369 100644
--- a/lib/cadet_web/controllers/session_controller.ex
+++ b/lib/cadet_web/controllers/session_controller.ex
@@ -45,7 +45,6 @@ defmodule CadetWeb.SessionController do
     case reason do
       :not_found -> "E-mail not registered in the system"
       :invalid_password -> "Invalid e-mail or password"
-      _ -> "Unknown"
     end
   end
 end
diff --git a/lib/mix/tasks/digest.ex b/lib/mix/tasks/digest.ex
index 424a98c92..19024189c 100644
--- a/lib/mix/tasks/digest.ex
+++ b/lib/mix/tasks/digest.ex
@@ -5,7 +5,11 @@ defmodule Mix.Tasks.Cadet.Digest do
   use Mix.Task
 
   def run(args) do
-    Dotenv.load!()
+    try do
+      Dotenv.load!()
+    rescue
+      e in RuntimeError -> e
+    end
     Mix.Shell.IO.cmd("cd frontend && npm run build")
     :ok = Mix.Tasks.Phx.Digest.run(args)
   end
diff --git a/lib/mix/tasks/server.ex b/lib/mix/tasks/server.ex
index f75c98489..bf28ee563 100644
--- a/lib/mix/tasks/server.ex
+++ b/lib/mix/tasks/server.ex
@@ -6,7 +6,11 @@ defmodule Mix.Tasks.Cadet.Server do
   use Mix.Task
 
   def run(args) do
-    Dotenv.load!()
+    try do
+      Dotenv.load!()
+    rescue
+      e in RuntimeError -> e
+    end
     :ok = Mix.Tasks.Phx.Server.run(args)
   end
 end
diff --git a/mix.exs b/mix.exs
index f4852fe31..d4df6ce2a 100644
--- a/mix.exs
+++ b/mix.exs
@@ -17,7 +17,8 @@ defmodule Cadet.Mixfile do
         "coveralls.html": :test
       ],
       aliases: aliases(),
-      deps: deps()
+      deps: deps(),
+      dialyzer: [plt_add_apps: [:mix, :ex_unit]]
     ]
   end
 
@@ -56,6 +57,7 @@ defmodule Cadet.Mixfile do
       {:phoenix_live_reload, "~> 1.0", only: :dev},
       {:credo, "~> 0.8", only: [:dev, :test], runtime: false},
       {:excoveralls, "~> 0.8", only: :test},
+      {:dialyxir, "~> 0.5", only: [:dev, :test], runtime: false},
       {:arc, "~> 0.8.0"},
       {:arc_ecto, "~> 0.7.0"},
       {:timex, "~> 3.0"},
diff --git a/mix.lock b/mix.lock
index 45a06ec69..25b8136ae 100644
--- a/mix.lock
+++ b/mix.lock
@@ -12,6 +12,7 @@
   "credo": {:hex, :credo, "0.9.1", "f021affa11b32a94dc2e807a6472ce0914289c9132f99644a97fc84432b202a1", [:mix], [{:bunt, "~> 0.2.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:poison, ">= 0.0.0", [hex: :poison, repo: "hexpm", optional: false]}], "hexpm"},
   "db_connection": {:hex, :db_connection, "1.1.2", "2865c2a4bae0714e2213a0ce60a1b12d76a6efba0c51fbda59c9ab8d1accc7a8", [], [{:connection, "~> 1.0.2", [hex: :connection, repo: "hexpm", optional: false]}, {:poolboy, "~> 1.5", [hex: :poolboy, repo: "hexpm", optional: true]}, {:sbroker, "~> 1.0", [hex: :sbroker, repo: "hexpm", optional: true]}], "hexpm"},
   "decimal": {:hex, :decimal, "1.4.1", "ad9e501edf7322f122f7fc151cce7c2a0c9ada96f2b0155b8a09a795c2029770", [], [], "hexpm"},
+  "dialyxir": {:hex, :dialyxir, "0.5.1", "b331b091720fd93e878137add264bac4f644e1ddae07a70bf7062c7862c4b952", [:mix], [], "hexpm"},
   "dotenv": {:hex, :dotenv, "2.0.0", "87e77a94e54c20eca49989876395e26878b109f016f9fd9c42b65bb9c59bca3f", [], [], "hexpm"},
   "ecto": {:hex, :ecto, "2.2.6", "3fd1067661d6d64851a0d4db9acd9e884c00d2d1aa41cc09da687226cf894661", [], [{:db_connection, "~> 1.1", [hex: :db_connection, repo: "hexpm", optional: true]}, {:decimal, "~> 1.2", [hex: :decimal, repo: "hexpm", optional: false]}, {:mariaex, "~> 0.8.0", [hex: :mariaex, repo: "hexpm", optional: true]}, {:poison, "~> 2.2 or ~> 3.0", [hex: :poison, repo: "hexpm", optional: true]}, {:poolboy, "~> 1.5", [hex: :poolboy, repo: "hexpm", optional: false]}, {:postgrex, "~> 0.13.0", [hex: :postgrex, repo: "hexpm", optional: true]}, {:sbroker, "~> 1.0", [hex: :sbroker, repo: "hexpm", optional: true]}], "hexpm"},
   "ecto_enum": {:hex, :ecto_enum, "1.1.0", "d44fe2ce6e1c0e907e7c3b6456a69e0f1d662348d8b4e2a662ba312223d8ff62", [], [{:ecto, ">= 2.0.0", [hex: :ecto, repo: "hexpm", optional: false]}, {:mariaex, ">= 0.0.0", [hex: :mariaex, repo: "hexpm", optional: true]}, {:postgrex, ">= 0.0.0", [hex: :postgrex, repo: "hexpm", optional: true]}], "hexpm"},
diff --git a/test/support/conn_case.ex b/test/support/conn_case.ex
index 62b70fca7..aaa043369 100644
--- a/test/support/conn_case.ex
+++ b/test/support/conn_case.ex
@@ -36,14 +36,11 @@ defmodule CadetWeb.ConnCase do
     conn = Phoenix.ConnTest.build_conn()
 
     if tags[:authenticate] != nil do
-      {:ok, conn: authenticate(conn, tags[:authenticate])}
+      user = Cadet.Factory.insert(:user, %{role: tags[:authenticate]})
+      conn = Cadet.Auth.Guardian.Plug.sign_in(conn, user)
+      {:ok, conn: conn}
     else
       {:ok, conn: conn}
     end
   end
-
-  defp authenticate(conn, role) do
-    user = Cadet.Factory.insert(:user, %{role: role})
-    Cadet.Auth.Guardian.Plug.sign_in(conn, user)
-  end
 end