diff --git a/module/CLI/src/Command/Shortcode/ResolveUrlCommand.php b/module/CLI/src/Command/Shortcode/ResolveUrlCommand.php
index 8c9af2fd..539035b1 100644
--- a/module/CLI/src/Command/Shortcode/ResolveUrlCommand.php
+++ b/module/CLI/src/Command/Shortcode/ResolveUrlCommand.php
@@ -71,14 +71,6 @@ class ResolveUrlCommand extends Command
 
         try {
             $longUrl = $this->urlShortener->shortCodeToUrl($shortCode);
-            if (! isset($longUrl)) {
-                $output->writeln(sprintf(
-                    '<error>' . $this->translator->translate('No URL found for short code "%s"') . '</error>',
-                    $shortCode
-                ));
-                return;
-            }
-
             $output->writeln(sprintf('%s <info>%s</info>', $this->translator->translate('Long URL:'), $longUrl));
         } catch (InvalidShortCodeException $e) {
             $output->writeln(sprintf('<error>' . $this->translator->translate(
diff --git a/module/Common/src/Exception/WrongIpException.php b/module/Common/src/Exception/WrongIpException.php
index 0e70d4e2..837f9ec7 100644
--- a/module/Common/src/Exception/WrongIpException.php
+++ b/module/Common/src/Exception/WrongIpException.php
@@ -5,7 +5,7 @@ namespace Shlinkio\Shlink\Common\Exception;
 
 class WrongIpException extends RuntimeException
 {
-    public static function fromIpAddress($ipAddress, \Exception $prev = null)
+    public static function fromIpAddress($ipAddress, \Throwable $prev = null)
     {
         return new self(sprintf('Provided IP "%s" is invalid', $ipAddress), 0, $prev);
     }
diff --git a/module/Common/src/Service/IpLocationResolver.php b/module/Common/src/Service/IpLocationResolver.php
index 218d2288..93b3ccfe 100644
--- a/module/Common/src/Service/IpLocationResolver.php
+++ b/module/Common/src/Service/IpLocationResolver.php
@@ -22,15 +22,17 @@ class IpLocationResolver implements IpLocationResolverInterface
     }
 
     /**
-     * @param $ipAddress
+     * @param string $ipAddress
      * @return array
+     * @throws WrongIpException
      */
-    public function resolveIpLocation($ipAddress)
+    public function resolveIpLocation(string $ipAddress): array
     {
         try {
             $response = $this->httpClient->get(sprintf(self::SERVICE_PATTERN, $ipAddress));
             return json_decode((string) $response->getBody(), true);
         } catch (GuzzleException $e) {
+            /** @var \Throwable $e */
             throw WrongIpException::fromIpAddress($ipAddress, $e);
         }
     }
diff --git a/module/Common/src/Service/IpLocationResolverInterface.php b/module/Common/src/Service/IpLocationResolverInterface.php
index 4f4279a2..fc1f07a3 100644
--- a/module/Common/src/Service/IpLocationResolverInterface.php
+++ b/module/Common/src/Service/IpLocationResolverInterface.php
@@ -3,11 +3,14 @@ declare(strict_types=1);
 
 namespace Shlinkio\Shlink\Common\Service;
 
+use Shlinkio\Shlink\Common\Exception\WrongIpException;
+
 interface IpLocationResolverInterface
 {
     /**
-     * @param $ipAddress
+     * @param string $ipAddress
      * @return array
+     * @throws WrongIpException
      */
-    public function resolveIpLocation($ipAddress);
+    public function resolveIpLocation(string $ipAddress): array;
 }
diff --git a/module/Common/src/Util/DateRange.php b/module/Common/src/Util/DateRange.php
index 215de520..f58f2033 100644
--- a/module/Common/src/Util/DateRange.php
+++ b/module/Common/src/Util/DateRange.php
@@ -6,11 +6,11 @@ namespace Shlinkio\Shlink\Common\Util;
 class DateRange
 {
     /**
-     * @var \DateTimeInterface
+     * @var \DateTimeInterface|null
      */
     private $startDate;
     /**
-     * @var \DateTimeInterface
+     * @var \DateTimeInterface|null
      */
     private $endDate;
 
@@ -21,7 +21,7 @@ class DateRange
     }
 
     /**
-     * @return \DateTimeInterface
+     * @return \DateTimeInterface|null
      */
     public function getStartDate()
     {
@@ -29,7 +29,7 @@ class DateRange
     }
 
     /**
-     * @return \DateTimeInterface
+     * @return \DateTimeInterface|null
      */
     public function getEndDate()
     {
@@ -39,8 +39,8 @@ class DateRange
     /**
      * @return bool
      */
-    public function isEmpty()
+    public function isEmpty(): bool
     {
-        return is_null($this->startDate) && is_null($this->endDate);
+        return $this->startDate === null && $this->endDate === null;
     }
 }
diff --git a/module/Core/src/Action/RedirectAction.php b/module/Core/src/Action/RedirectAction.php
index e22639c7..066e9508 100644
--- a/module/Core/src/Action/RedirectAction.php
+++ b/module/Core/src/Action/RedirectAction.php
@@ -49,12 +49,6 @@ class RedirectAction implements MiddlewareInterface
         try {
             $longUrl = $this->urlShortener->shortCodeToUrl($shortCode);
 
-            // If provided shortCode does not belong to a valid long URL, dispatch next middleware, which will trigger
-            // a not-found error
-            if ($longUrl === null) {
-                return $delegate->process($request);
-            }
-
             // Track visit to this short code
             $this->visitTracker->track($shortCode, $request);
 
diff --git a/module/Core/src/Exception/InvalidUrlException.php b/module/Core/src/Exception/InvalidUrlException.php
index 848cd71b..13e77c5d 100644
--- a/module/Core/src/Exception/InvalidUrlException.php
+++ b/module/Core/src/Exception/InvalidUrlException.php
@@ -7,9 +7,9 @@ use Shlinkio\Shlink\Common\Exception\RuntimeException;
 
 class InvalidUrlException extends RuntimeException
 {
-    public static function fromUrl($url, \Exception $previous = null)
+    public static function fromUrl($url, \Throwable $previous = null)
     {
         $code = isset($previous) ? $previous->getCode() : -1;
-        return new static(sprintf('Provided URL "%s" is not an exisitng and valid URL', $url), $code, $previous);
+        return new static(sprintf('Provided URL "%s" is not an existing and valid URL', $url), $code, $previous);
     }
 }
diff --git a/module/Core/src/Service/ShortUrlService.php b/module/Core/src/Service/ShortUrlService.php
index 2e22717b..85359060 100644
--- a/module/Core/src/Service/ShortUrlService.php
+++ b/module/Core/src/Service/ShortUrlService.php
@@ -49,9 +49,9 @@ class ShortUrlService implements ShortUrlServiceInterface
      * @return ShortUrl
      * @throws InvalidShortCodeException
      */
-    public function setTagsByShortCode($shortCode, array $tags = [])
+    public function setTagsByShortCode($shortCode, array $tags = []): ShortUrl
     {
-        /** @var ShortUrl $shortUrl */
+        /** @var ShortUrl|null $shortUrl */
         $shortUrl = $this->em->getRepository(ShortUrl::class)->findOneBy([
             'shortCode' => $shortCode,
         ]);
diff --git a/module/Core/src/Service/ShortUrlServiceInterface.php b/module/Core/src/Service/ShortUrlServiceInterface.php
index e299842d..1815c4de 100644
--- a/module/Core/src/Service/ShortUrlServiceInterface.php
+++ b/module/Core/src/Service/ShortUrlServiceInterface.php
@@ -24,5 +24,5 @@ interface ShortUrlServiceInterface
      * @return ShortUrl
      * @throws InvalidShortCodeException
      */
-    public function setTagsByShortCode($shortCode, array $tags = []);
+    public function setTagsByShortCode($shortCode, array $tags = []): ShortUrl;
 }
diff --git a/module/Core/src/Service/Tag/TagService.php b/module/Core/src/Service/Tag/TagService.php
index b9427eee..247112c8 100644
--- a/module/Core/src/Service/Tag/TagService.php
+++ b/module/Core/src/Service/Tag/TagService.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 namespace Shlinkio\Shlink\Core\Service\Tag;
 
 use Doctrine\Common\Collections\Collection;
+use Doctrine\ORM\EntityManager;
 use Doctrine\ORM\EntityManagerInterface;
 use Shlinkio\Shlink\Core\Entity\Tag;
 use Shlinkio\Shlink\Core\Exception\EntityDoesNotExistException;
@@ -15,7 +16,7 @@ class TagService implements TagServiceInterface
     use TagManagerTrait;
 
     /**
-     * @var EntityManagerInterface
+     * @var EntityManager|EntityManagerInterface
      */
     private $em;
 
diff --git a/module/Core/src/Service/UrlShortener.php b/module/Core/src/Service/UrlShortener.php
index cd282035..6515fd0f 100644
--- a/module/Core/src/Service/UrlShortener.php
+++ b/module/Core/src/Service/UrlShortener.php
@@ -90,6 +90,7 @@ class UrlShortener implements UrlShortenerInterface
         int $maxVisits = null
     ): string {
         // If the url already exists in the database, just return its short code
+        /** @var ShortUrl|null $shortUrl */
         $shortUrl = $this->em->getRepository(ShortUrl::class)->findOneBy([
             'originalUrl' => $url,
         ]);
@@ -148,6 +149,7 @@ class UrlShortener implements UrlShortenerInterface
                 'max' => 15,
             ]]);
         } catch (GuzzleException $e) {
+            /** @var \Throwable $e */
             throw InvalidUrlException::fromUrl($url, $e);
         }
     }
@@ -155,13 +157,13 @@ class UrlShortener implements UrlShortenerInterface
     /**
      * Generates the unique shortcode for an autoincrement ID
      *
-     * @param int $id
+     * @param float $id
      * @return string
      */
-    private function convertAutoincrementIdToShortCode($id): string
+    private function convertAutoincrementIdToShortCode(float $id): string
     {
-        $id = ((int) $id) + 200000; // Increment the Id so that the generated shortcode is not too short
-        $length = strlen($this->chars);
+        $id += 200000; // Increment the Id so that the generated shortcode is not too short
+        $length = \strlen($this->chars);
         $code = '';
 
         while ($id > 0) {
diff --git a/module/Core/src/Service/VisitsTracker.php b/module/Core/src/Service/VisitsTracker.php
index 38e44fea..6147046b 100644
--- a/module/Core/src/Service/VisitsTracker.php
+++ b/module/Core/src/Service/VisitsTracker.php
@@ -15,7 +15,7 @@ use Shlinkio\Shlink\Core\Repository\VisitRepository;
 class VisitsTracker implements VisitsTrackerInterface
 {
     /**
-     * @var EntityManagerInterface|EntityManager
+     * @var EntityManager|EntityManagerInterface
      */
     private $em;
 
@@ -66,14 +66,14 @@ class VisitsTracker implements VisitsTrackerInterface
     /**
      * Returns the visits on certain short code
      *
-     * @param $shortCode
+     * @param string $shortCode
      * @param DateRange $dateRange
      * @return Visit[]
      * @throws InvalidArgumentException
      */
-    public function info($shortCode, DateRange $dateRange = null): array
+    public function info(string $shortCode, DateRange $dateRange = null): array
     {
-        /** @var ShortUrl $shortUrl */
+        /** @var ShortUrl|null $shortUrl */
         $shortUrl = $this->em->getRepository(ShortUrl::class)->findOneBy([
             'shortCode' => $shortCode,
         ]);
diff --git a/module/Core/src/Service/VisitsTrackerInterface.php b/module/Core/src/Service/VisitsTrackerInterface.php
index d3295f37..a852c030 100644
--- a/module/Core/src/Service/VisitsTrackerInterface.php
+++ b/module/Core/src/Service/VisitsTrackerInterface.php
@@ -21,10 +21,10 @@ interface VisitsTrackerInterface
     /**
      * Returns the visits on certain short code
      *
-     * @param $shortCode
+     * @param string $shortCode
      * @param DateRange $dateRange
      * @return Visit[]
      * @throws InvalidArgumentException
      */
-    public function info($shortCode, DateRange $dateRange = null): array;
+    public function info(string $shortCode, DateRange $dateRange = null): array;
 }
diff --git a/module/Rest/src/Action/CreateShortcodeAction.php b/module/Rest/src/Action/CreateShortcodeAction.php
index 8edcd499..56169592 100644
--- a/module/Rest/src/Action/CreateShortcodeAction.php
+++ b/module/Rest/src/Action/CreateShortcodeAction.php
@@ -50,7 +50,7 @@ class CreateShortcodeAction extends AbstractRestAction
      */
     public function process(Request $request, DelegateInterface $delegate)
     {
-        $postData = $request->getParsedBody();
+        $postData = (array) $request->getParsedBody();
         if (! isset($postData['longUrl'])) {
             return new JsonResponse([
                 'error' => RestUtils::INVALID_ARGUMENT_ERROR,
diff --git a/module/Rest/src/Action/GetVisitsAction.php b/module/Rest/src/Action/GetVisitsAction.php
index 0d03acf1..490b0470 100644
--- a/module/Rest/src/Action/GetVisitsAction.php
+++ b/module/Rest/src/Action/GetVisitsAction.php
@@ -75,10 +75,10 @@ class GetVisitsAction extends AbstractRestAction
 
     /**
      * @param Request $request
-     * @param $key
+     * @param string $key
      * @return \DateTime|null
      */
-    protected function getDateQueryParam(Request $request, $key)
+    private function getDateQueryParam(Request $request, string $key)
     {
         $query = $request->getQueryParams();
         if (! isset($query[$key]) || empty($query[$key])) {
diff --git a/module/Rest/src/Authentication/JWTService.php b/module/Rest/src/Authentication/JWTService.php
index 74130438..6a0d3ac8 100644
--- a/module/Rest/src/Authentication/JWTService.php
+++ b/module/Rest/src/Authentication/JWTService.php
@@ -27,7 +27,7 @@ class JWTService implements JWTServiceInterface
      * @param int $lifetime
      * @return string
      */
-    public function create(ApiKey $apiKey, $lifetime = self::DEFAULT_LIFETIME)
+    public function create(ApiKey $apiKey, $lifetime = self::DEFAULT_LIFETIME): string
     {
         $currentTimestamp = time();
 
@@ -48,7 +48,7 @@ class JWTService implements JWTServiceInterface
      * @return string
      * @throws AuthenticationException If the token has expired
      */
-    public function refresh($jwt, $lifetime = self::DEFAULT_LIFETIME)
+    public function refresh(string $jwt, $lifetime = self::DEFAULT_LIFETIME): string
     {
         $payload = $this->getPayload($jwt);
         $payload['exp'] = time() + $lifetime;
@@ -61,7 +61,7 @@ class JWTService implements JWTServiceInterface
      * @param string $jwt
      * @return bool
      */
-    public function verify($jwt)
+    public function verify(string $jwt): bool
     {
         try {
             // If no exception is thrown while decoding the token, it is considered valid
@@ -79,7 +79,7 @@ class JWTService implements JWTServiceInterface
      * @return array
      * @throws AuthenticationException If the token has expired
      */
-    public function getPayload($jwt)
+    public function getPayload(string $jwt): array
     {
         try {
             return $this->decode($jwt);
@@ -92,16 +92,16 @@ class JWTService implements JWTServiceInterface
      * @param array $data
      * @return string
      */
-    protected function encode(array $data)
+    protected function encode(array $data): string
     {
         return JWT::encode($data, $this->appOptions->getSecretKey(), self::DEFAULT_ENCRYPTION_ALG);
     }
 
     /**
-     * @param $jwt
+     * @param string $jwt
      * @return array
      */
-    protected function decode($jwt)
+    protected function decode(string $jwt): array
     {
         return (array) JWT::decode($jwt, $this->appOptions->getSecretKey(), [self::DEFAULT_ENCRYPTION_ALG]);
     }
diff --git a/module/Rest/src/Authentication/JWTServiceInterface.php b/module/Rest/src/Authentication/JWTServiceInterface.php
index 55c8fb36..ba162aa8 100644
--- a/module/Rest/src/Authentication/JWTServiceInterface.php
+++ b/module/Rest/src/Authentication/JWTServiceInterface.php
@@ -18,7 +18,7 @@ interface JWTServiceInterface
      * @param int $lifetime
      * @return string
      */
-    public function create(ApiKey $apiKey, $lifetime = self::DEFAULT_LIFETIME);
+    public function create(ApiKey $apiKey, $lifetime = self::DEFAULT_LIFETIME): string;
 
     /**
      * Refreshes a token and returns it with the new expiration
@@ -28,7 +28,7 @@ interface JWTServiceInterface
      * @return string
      * @throws AuthenticationException If the token has expired
      */
-    public function refresh($jwt, $lifetime = self::DEFAULT_LIFETIME);
+    public function refresh(string $jwt, $lifetime = self::DEFAULT_LIFETIME): string;
 
     /**
      * Verifies that certain JWT is valid
@@ -36,7 +36,7 @@ interface JWTServiceInterface
      * @param string $jwt
      * @return bool
      */
-    public function verify($jwt);
+    public function verify(string $jwt): bool;
 
     /**
      * Decodes certain token and returns the payload
@@ -45,5 +45,5 @@ interface JWTServiceInterface
      * @return array
      * @throws AuthenticationException If the token has expired
      */
-    public function getPayload($jwt);
+    public function getPayload(string $jwt): array;
 }
diff --git a/module/Rest/src/Middleware/CheckAuthenticationMiddleware.php b/module/Rest/src/Middleware/CheckAuthenticationMiddleware.php
index b5823f4f..071eadba 100644
--- a/module/Rest/src/Middleware/CheckAuthenticationMiddleware.php
+++ b/module/Rest/src/Middleware/CheckAuthenticationMiddleware.php
@@ -55,13 +55,14 @@ class CheckAuthenticationMiddleware implements MiddlewareInterface, StatusCodeIn
      *
      * @return Response
      * @throws \InvalidArgumentException
+     * @throws \ErrorException
      */
     public function process(Request $request, DelegateInterface $delegate)
     {
         // If current route is the authenticate route or an OPTIONS request, continue to the next middleware
-        /** @var RouteResult $routeResult */
+        /** @var RouteResult|null $routeResult */
         $routeResult = $request->getAttribute(RouteResult::class);
-        if (! isset($routeResult)
+        if ($routeResult === null
             || $routeResult->isFailure()
             || $routeResult->getMatchedRouteName() === AuthenticateAction::class
             || $request->getMethod() === 'OPTIONS'
diff --git a/module/Rest/src/Service/ApiKeyService.php b/module/Rest/src/Service/ApiKeyService.php
index e2473f1b..6f368e56 100644
--- a/module/Rest/src/Service/ApiKeyService.php
+++ b/module/Rest/src/Service/ApiKeyService.php
@@ -46,13 +46,9 @@ class ApiKeyService implements ApiKeyServiceInterface
      */
     public function check($key)
     {
-        /** @var ApiKey $apiKey */
+        /** @var ApiKey|null $apiKey */
         $apiKey = $this->getByKey($key);
-        if (! isset($apiKey)) {
-            return false;
-        }
-
-        return $apiKey->isValid();
+        return $apiKey !== null && $apiKey->isValid();
     }
 
     /**
@@ -60,12 +56,13 @@ class ApiKeyService implements ApiKeyServiceInterface
      *
      * @param string $key
      * @return ApiKey
+     * @throws InvalidArgumentException
      */
     public function disable($key)
     {
-        /** @var ApiKey $apiKey */
+        /** @var ApiKey|null $apiKey */
         $apiKey = $this->getByKey($key);
-        if (! isset($apiKey)) {
+        if ($apiKey === null) {
             throw new InvalidArgumentException(sprintf('API key "%s" does not exist and can\'t be disabled', $key));
         }
 
@@ -75,7 +72,7 @@ class ApiKeyService implements ApiKeyServiceInterface
     }
 
     /**
-     * Lists all existing appi keys
+     * Lists all existing api keys
      *
      * @param bool $enabledOnly Tells if only enabled keys should be returned
      * @return ApiKey[]
@@ -94,8 +91,10 @@ class ApiKeyService implements ApiKeyServiceInterface
      */
     public function getByKey($key)
     {
-        return $this->em->getRepository(ApiKey::class)->findOneBy([
+        /** @var ApiKey|null $apiKey */
+        $apiKey = $this->em->getRepository(ApiKey::class)->findOneBy([
             'key' => $key,
         ]);
+        return $apiKey;
     }
 }
diff --git a/module/Rest/src/Service/ApiKeyServiceInterface.php b/module/Rest/src/Service/ApiKeyServiceInterface.php
index 51dadf7a..ceaa3b96 100644
--- a/module/Rest/src/Service/ApiKeyServiceInterface.php
+++ b/module/Rest/src/Service/ApiKeyServiceInterface.php
@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Shlinkio\Shlink\Rest\Service;
 
+use Shlinkio\Shlink\Common\Exception\InvalidArgumentException;
 use Shlinkio\Shlink\Rest\Entity\ApiKey;
 
 interface ApiKeyServiceInterface
@@ -28,11 +29,12 @@ interface ApiKeyServiceInterface
      *
      * @param string $key
      * @return ApiKey
+     * @throws InvalidArgumentException
      */
     public function disable($key);
 
     /**
-     * Lists all existing appi keys
+     * Lists all existing api keys
      *
      * @param bool $enabledOnly Tells if only enabled keys should be returned
      * @return ApiKey[]
diff --git a/module/Rest/test/Action/AuthenticateActionTest.php b/module/Rest/test/Action/AuthenticateActionTest.php
index 32079e9b..4fb2518f 100644
--- a/module/Rest/test/Action/AuthenticateActionTest.php
+++ b/module/Rest/test/Action/AuthenticateActionTest.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 namespace ShlinkioTest\Shlink\Rest\Action;
 
 use PHPUnit\Framework\TestCase;
+use Prophecy\Argument;
 use Prophecy\Prophecy\ObjectProphecy;
 use Shlinkio\Shlink\Rest\Action\AuthenticateAction;
 use Shlinkio\Shlink\Rest\Authentication\JWTService;
@@ -32,6 +33,8 @@ class AuthenticateActionTest extends TestCase
     {
         $this->apiKeyService = $this->prophesize(ApiKeyService::class);
         $this->jwtService = $this->prophesize(JWTService::class);
+        $this->jwtService->create(Argument::cetera())->willReturn('');
+
         $this->action = new AuthenticateAction(
             $this->apiKeyService->reveal(),
             $this->jwtService->reveal(),