diff --git a/.travis.yml b/.travis.yml
index 43c20ef..e211c37 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,6 +12,7 @@ before_script:
 
 script:
   - vendor/bin/phpunit --coverage-clover=coverage.xml
+  - composer phpstan
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)
diff --git a/README.md b/README.md
index da27dc1..1823a09 100644
--- a/README.md
+++ b/README.md
@@ -62,9 +62,10 @@ npm run serve
 Once everything is ready visit: ```http://localhost:8080```
 
 
-### Run tests
+### Run tests & static analysis
 ```
 vendor/bin/phpunit
+composer phpstan
 ```
 
 ### Deployment
diff --git a/backend/Controllers/AuthController.php b/backend/Controllers/AuthController.php
index de248a5..dd4b0e2 100644
--- a/backend/Controllers/AuthController.php
+++ b/backend/Controllers/AuthController.php
@@ -67,6 +67,10 @@ class AuthController
             return $response->json($errors->firstOfAll(), 422);
         }
 
+        if ($auth->user() === null) {
+            return $response->json(['oldpassword' => 'Wrong password'], 422);
+        }
+
         if (! $auth->authenticate($auth->user()->getUsername(), $request->input('oldpassword'))) {
             return $response->json(['oldpassword' => 'Wrong password'], 422);
         }
diff --git a/backend/Services/Auth/Adapters/Database.php b/backend/Services/Auth/Adapters/Database.php
index fcb5471..d9a716e 100644
--- a/backend/Services/Auth/Adapters/Database.php
+++ b/backend/Services/Auth/Adapters/Database.php
@@ -101,7 +101,7 @@ class Database implements Service, AuthInterface
             ], 'WHERE username = ?', $username);
         }
 
-        return $this->find($user->getUsername());
+        return $this->find($user->getUsername()) ?: $user;
     }
 
     public function add(User $user, $password): User
@@ -119,7 +119,7 @@ class Database implements Service, AuthInterface
             'password' => $this->hashPassword($password),
         ]);
 
-        return $this->find($user->getUsername());
+        return $this->find($user->getUsername()) ?: $user;
     }
 
     public function delete(User $user)
diff --git a/backend/Services/Auth/Adapters/JsonFile.php b/backend/Services/Auth/Adapters/JsonFile.php
index 381ec00..0bc4510 100644
--- a/backend/Services/Auth/Adapters/JsonFile.php
+++ b/backend/Services/Auth/Adapters/JsonFile.php
@@ -96,7 +96,7 @@ class JsonFile implements Service, AuthInterface
 
                 $this->saveUsers($all_users);
 
-                return $this->find($user->getUsername());
+                return $this->find($user->getUsername()) ?: $user;
             }
         }
 
@@ -122,7 +122,7 @@ class JsonFile implements Service, AuthInterface
 
         $this->saveUsers($all_users);
 
-        return $this->find($user->getUsername());
+        return $this->find($user->getUsername()) ?: $user;
     }
 
     public function delete(User $user)
diff --git a/backend/Services/Session/Adapters/SessionStorage.php b/backend/Services/Session/Adapters/SessionStorage.php
index 9bb0a65..34c1256 100644
--- a/backend/Services/Session/Adapters/SessionStorage.php
+++ b/backend/Services/Session/Adapters/SessionStorage.php
@@ -55,7 +55,11 @@ class SessionStorage implements Service, SessionStorageInterface
 
     public function invalidate()
     {
-        if ($this->getSession() !== null || ! $this->getSession()->isStarted()) {
+        if ($this->getSession() === null) {
+            return;
+        }
+
+        if (! $this->getSession()->isStarted()) {
             $this->getSession()->start();
         }
 
diff --git a/backend/Services/Storage/Filesystem.php b/backend/Services/Storage/Filesystem.php
index 48f01f4..aa6526b 100644
--- a/backend/Services/Storage/Filesystem.php
+++ b/backend/Services/Storage/Filesystem.php
@@ -282,6 +282,6 @@ class Filesystem implements Service
 
         $tmp = explode($this->separator, trim($path, $this->separator));
 
-        return  array_pop($tmp);
+        return  (string) array_pop($tmp);
     }
 }
diff --git a/composer.json b/composer.json
index 5f0b60b..79b0875 100644
--- a/composer.json
+++ b/composer.json
@@ -36,5 +36,8 @@
         "symfony/var-dumper": "^4.2",
         "league/flysystem-memory": "^1.0",
         "phpstan/phpstan": "^0.11.8"
+    },
+    "scripts": {
+        "phpstan": "phpstan analyse ./backend --level=7"
     }
 }